MdeModulePkg/RegularExpressionDxe:disable wraning to pass gcc4.8 build

There are three warnings reported by GCC 4.8 and the later GCC release
are workaround with them.
And all the three warnings are invalid,so I just disable warnings rather
than fix them at now.

Following is the analysis from Laszlo Ersek.
(1)

> MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regcomp.c: In
> function 'compile_length_tree':
> MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regcomp.c:1516:7:
> warning: 'len' may be used uninitialized in this function
> [-Wmaybe-uninitialized]
>    int len;
>        ^

I think this is an invalid warning; the type of the controlling expression
(node->type) is enum GimmickType, and the case labels cover all values of
the enum. An assert(0) could be added, I guess, but again, upstream
Oniguruma would be justified to reject the idea.

(2)

> MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regparse.c: In
> function 'parse_callout_args.isra.10.constprop.30':
> MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regparse.c:6753:25:
> warning: 'rl' may be used uninitialized in this function
> [-Wmaybe-uninitialized]
>   vals[n].l = rl;
>               ^

This warning is invalid, given:

  6749    if (cn > 0) {
  6750      long rl;
  6751      r = parse_long(enc, buf, bufend, 1, LONG_MAX, &rl);
  6752      if (r == ONIG_NORMAL) {
  6753        vals[n].l = rl;

Because parse_long() only returns ONIG_NORMAL after it sets (*rl).

(3)

> MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regparse.c: In
> function 'parse_callout_of_name.constprop.29':
> MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regparse.c:6861:38:
> warning: 'tag_end' may be used uninitialized in this function
> [-Wmaybe-uninitialized]
>      if (! is_allowed_callout_tag_name(enc, tag_start, tag_end))

This warning is also invalid, given:

  6852    if (c == '[') {
  6853      if (PEND) return ONIGERR_END_PATTERN_IN_GROUP;
  6854      tag_start = p;
  6855      while (! PEND) {
  6856        if (PEND) return ONIGERR_END_PATTERN_IN_GROUP;
  6857        tag_end = p;
  6858        PFETCH_S(c);
  6859        if (c == ']') break;
  6860      }
  6861      if (! is_allowed_callout_tag_name(enc, tag_start, tag_end))
  6862        return ONIGERR_INVALID_CALLOUT_TAG_NAME;
  6863

To see that, first we should note:
 #define PEND         (p < end ?  0 : 1)

therefore PEND doesn't change if neither "p" nor "end" change.

Second, when we reach line 6855 (the "while") for the very first time,
(!PEND) is certainly true (i.e., PEND is false), because otherwise we
would have bailed at line 6853. Therefore we reach line 6857, and assign
"tag_end". Regardless of whether we iterate zero or more *additional*
times around the loop, "tag_end" will have been set, whenever we reach
line 6861.

Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dongao Guo <dongao.guo@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

MdeModulePkg/Universal/RegularExpressionDxe/RegularExpressionDxe.inf

@@ -109,3 +109,6 @@
 
   # Oniguruma: error: variable 'fp' set but not used
   GCC:*_*_*_CC_FLAGS = -Wno-error=unused-but-set-variable
+
+  # Oniguruma: tag_end in parse_callout_of_name
+  GCC:*_*_*_CC_FLAGS = -Wno-error=maybe-uninitialized