Assume <errno.h> exists

ISO C99 requires <errno.h>.

Many files in the project already include <errno.h> unconditionally,
so it's reasonable to assume that it is always available.

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

@@ -48,9 +48,7 @@ extern char * textdomain (const char * domainname);
 #include <stdlib.h>
 #include <string.h>
 
-#if HAVE_ERRNO_H
-# include <errno.h>
-#endif
+#include <errno.h>
 
 #include <sys/stat.h>
 #include <sys/types.h>