aix: Resolve build failure with default C23

aix: Resolve build failure with default C23

libiberty/getopt.c file is defining _NO_PROTO, which causes
conflicting declarations for the functions in AIX header files
like stdio.h & stdlib.h.
Looks like _NO_PROTO define were added long back and conflicting
declarations were always present until C23 standard uncovered it.

Remove the block defining _NO_PROTO as both Tru64 UNIX (ex-OSF/1)
and AIX 3.2 is no more supported.

libiberty/ChangeLog:

	* getopt.c: Remove _NO_PROTO block