Assume strerror(3) exists

ISO C99 requires strerror(3).

Link: <https://github.com/shadow-maint/shadow/pull/600>
Cc: Christian Göttsche <cgzones@googlemail.com>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/defines.h

@@ -250,7 +250,6 @@ static inline void memzero(void *ptr, size_t size)
 #ifdef sun			/* hacks for compiling on SunOS */
 # ifndef SOLARIS
 extern char *strdup ();
-extern char *strerror ();
 # endif
 #endif