Put brackets around mblen() compat constant.

This might help with the reported problem cross compiling for Android
("error: expected identifier or '(' before numeric constant") but
shouldn't hurt in any case.

openbsd-compat/openbsd-compat.h

@@ -221,7 +221,7 @@ long long strtonum(const char *, long long, long long, const char **);
 
 /* multibyte character support */
 #ifndef HAVE_MBLEN
-# define mblen(x, y)	1
+# define mblen(x, y)	(1)
 #endif
 
 #if !defined(HAVE_VASPRINTF) || !defined(HAVE_VSNPRINTF)