Always take GCC's definition of NULL.

There's a (bad) definition of NULL in <linux/stddef.h>, and this
definition of NULL, and the One True definition in the <stddef.h> provided
by the compiler. This change at least kills one of the bad duplicates.
Killing the <linux/stddef.h> one is harder, because it's in a generated
file.

Change-Id: Iea4ccb12d6758199f312ea9cd753b84322d5c471
diff --git a/libc/include/stdio.h b/libc/include/stdio.h
index d7c881c..3f453fe 100644
--- a/libc/include/stdio.h
+++ b/libc/include/stdio.h
@@ -63,13 +63,8 @@
 typedef	long    off_t;
 #endif
 
-#ifndef NULL
-#ifdef 	__GNUG__
-#define	NULL	__null
-#else
-#define	NULL	0L
-#endif
-#endif
+#define __need_NULL
+#include <stddef.h>
 
 #define	_FSTDIO			/* Define for new stdio with functions. */