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-gcc.h

@@ -294,6 +294,14 @@
 #define __no_sanitize_address __attribute__((no_sanitize_address))
 #endif
 
+#if GCC_VERSION >= 50100
+/*
+ * Mark structures as requiring designated initializers.
+ * https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html
+ */
+#define __designated_init __attribute__((designated_init))
+#endif
+
 #endif	/* gcc version >= 40000 specific checks */
 
 #if !defined(__noclone)