configure: Fix finding xvid by including stddef.h for definition of NULL

[ryandesign]

The configure test for xvid was failing to find it due to

*** CC/CXX Test Failed (res 1 args -L/opt/local/lib -Wl,-headerpad_max_install_names -Wl,-syslibroot,/Library/Developer/CommandLineTools/SDKs/MacOSX12.sdk -arch x86_64 -arch x86_64 -L/opt/local/lib -lxvidcore -lpthread) : 

/opt/local/var/macports/build/_Volumes_Shared_macports-ports_multimedia_gpac/gpac/work/.tmp/gpac-conf-30001-60580-19987.c:2:70: error: use of undeclared identifier 'NULL'
int main( void ) { void *codec; xvid_decore(codec, XVID_DEC_DESTROY, NULL, NULL); return 0; }
                                                                     ^
/opt/local/var/macports/build/_Volumes_Shared_macports-ports_multimedia_gpac/gpac/work/.tmp/gpac-conf-30001-60580-19987.c:2:76: error: use of undeclared identifier 'NULL'
int main( void ) { void *codec; xvid_decore(codec, XVID_DEC_DESTROY, NULL, NULL); return 0; }
                                                                           ^
2 errors generated.

Source was: 
#include <xvid.h>
int main( void ) { void *codec; xvid_decore(codec, XVID_DEC_DESTROY, NULL, NULL); return 0; }

Fix by including <stddef.h> anytime NULL is referenced.

[rbouqueau]

Thank you! LGTM, merging.