expose memmem under baseline POSIX feature profile

memmem has been adopted for the next issue of POSIX (outcome of
tracker item 1061). since mem* is in the reserved namespace for
string.h it's already fully conforming to expose it by default, so
just do so.

include/string.h

@@ -73,6 +73,7 @@ char *strsignal(int);
 char *strerror_l (int, locale_t);
 int strcoll_l (const char *, const char *, locale_t);
 size_t strxfrm_l (char *__restrict, const char *__restrict, size_t, locale_t);
+void *memmem(const void *, size_t, const void *, size_t);
 #endif
 
 #if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) \
@@ -92,7 +93,6 @@ void explicit_bzero (void *, size_t);
 int strverscmp (const char *, const char *);
 char *strchrnul(const char *, int);
 char *strcasestr(const char *, const char *);
-void *memmem(const void *, size_t, const void *, size_t);
 void *memrchr(const void *, int, size_t);
 void *mempcpy(void *, const void *, size_t);
 #ifndef __cplusplus