Move <string.h> inclusion to compiler.h

There is absolutely no reason not to include <string.h> globally, and
with the inline function for mempcpy() we need it there anyway.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

stdlib/vsnprintf.c

@@ -10,7 +10,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdarg.h>
-#include <string.h>
 
 #include "nasmlib.h"
 #include "error.h"