include/uapi/linux/agpgart.h: include stdlib.h in userspace

Fixes userspace compiler error:
error: unknown type name ‘size_t’
Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>

include/uapi/linux/agpgart.h

@@ -52,6 +52,7 @@
 
 #ifndef __KERNEL__
 #include <linux/types.h>
+#include <stdlib.h>
 
 struct agp_version {
 	__u16 major;