os.h: drop unnecessary guard on stdlib.h include

It's a standard C header, that's always present, so no extra guard needed.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1684>

include/os.h

@@ -50,9 +50,7 @@ SOFTWARE.
 #include "misc.h"
 #include <stdarg.h>
 #include <stdint.h>
-#if defined(HAVE_REALLOCARRAY)
-#include <stdlib.h>       /* for reallocarray */
-#endif
+#include <stdlib.h>
 #include <string.h>
 #ifdef MONOTONIC_CLOCK
 #include <time.h>