lsefi: Add missing static qualifier

known_protocols isn't used anywhere else and even misses grub_ prefix, so
let's make it local (static).

Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/commands/efi/lsefi.c

@@ -29,7 +29,7 @@
 
 GRUB_MOD_LICENSE ("GPLv3+");
 
-struct known_protocol
+static struct known_protocol
 {
   grub_guid_t guid;
   const char *name;