Add fake '__builtin_warning()' for the gcc case.

Allows us to do compile-time sparse warnings of our own.

include/linux/compiler.h

@@ -17,6 +17,7 @@ extern void __chk_io_ptr(void __iomem *);
 # define __iomem
 # define __chk_user_ptr(x) (void)0
 # define __chk_io_ptr(x) (void)0
+#define __builtin_warning(x, ...) (1)
 #endif
 
 #ifdef __KERNEL__