Build fixes for argp.h with older gcc.

util/grub-mkstandalone.c

@@ -22,9 +22,14 @@
 #include <grub/util/misc.h>
 #include <grub/emu/config.h>
 
-#include <argp.h>
 #include <string.h>
 
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-declarations"
+#include <argp.h>
+#pragma GCC diagnostic error "-Wmissing-prototypes"
+#pragma GCC diagnostic error "-Wmissing-declarations"
+
 static char *output_image;
 static char **files;
 static int nfiles;