util/Compiler.h: add gcc_fallthrough

Works around build failures with ccache which may feed processed code
to GCC, which doesn't have the "fall through" code comments.

src/util/format.c

@@ -19,6 +19,7 @@
  */
 
 #include "format.h"
+#include "util/Compiler.h"
 
 #include <stdbool.h>
 #include <stdio.h>
@@ -238,6 +239,7 @@ format_object2(const char *format, const char **last, const void *object,
 			}
 
 			/* fall through */
+			gcc_fallthrough;
 
 		default:
 			/* pass-through non-escaped portions of the format string */