compat/fnmatch/fnmatch.c: give a fall-back definition for NULL

Somebody tried to compile fnmatch.c compatibility file on Interix and got an error because no header included in the file on that platform defined NULL. It usually comes from stddef.h and indirectly from other headers like string.h, unistd.h, stdio.h, stdlib.h, etc., but with the way we compile this file from our Makefile, inclusion of the header files that are expected to define NULL in fnmatch.c do not happen because they are protected with "#ifdef STDC_HEADERS", etc. which we do not pass. As the least-impact workaround, give a fall-back definition when none of the headers define NULL. Noticed-by: Markus Duft <mduft@gentoo.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>