realloc-posix: Fix link error (regression yesterday). * lib/stdlib.in.h (realloc): Don't inline rpl_realloc if IN_GNULIB_TESTS is defined. * modules/realloc-posix (Depends-on, configure.ac): Compile realloc.c also if REPLACE_REALLOC_FOR_REALLOC_POSIX is 2.