BaseTools/Pccts: Resolve GCC sting format mismatch build warning

https://bugzilla.tianocore.org/show_bug.cgi?id=282

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/C/VfrCompile/Pccts/support/set/set.c

@@ -554,7 +554,7 @@ register char *s;
 	static set a;
 	register unsigned *p, *endp;
 
-	set_new(a, strlen(s));
+	set_new(a, (unsigned) strlen(s));
 	p = a.setword;
 	endp = &(a.setword[a.n]);
 	do {