compiler: Add __designated_init annotation

This allows structure annotations for requiring designated initialization
in GCC 5.1.0 and later:
https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html

The structure randomization layout plugin will be using this to help
identify structures that need this form of initialization.

Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/compiler.h

@@ -440,6 +440,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 # define __attribute_const__	/* unimplemented */
 #endif
 
+#ifndef __designated_init
+# define __designated_init
+#endif
+
 #ifndef __latent_entropy
 # define __latent_entropy
 #endif