The `<stddef.h>` header file is part of the standard C89 headers [1] and
on current systems there is no need for a manual check if header is
present.
Since PHP requires at least C89 the `HAVE_STDDEF_H` symbol isn't defined
by Autoconf anywhere else anymore [2] and accross the PHP source code
the header is included unconditionally already.
This patch syncs this also for the bundled libmbfl which is maintaned as
a fork in php-src.
Refs:
[1] https://port70.net/~nsz/c/c89/c89-draft.html#4.1.2
[2] https://git.savannah.gnu.org/cgit/autoconf.git/tree/lib/autoconf/headers.m4
6c1ff61a36