disk: efi: GUIDs should be const

Make system_guid const.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/disk/part_efi.c b/disk/part_efi.c
index d6bb53e..239455b 100644
--- a/disk/part_efi.c
+++ b/disk/part_efi.c
@@ -66,7 +66,7 @@
 	return name;
 }
 
-static efi_guid_t system_guid = PARTITION_SYSTEM_GUID;
+static const efi_guid_t system_guid = PARTITION_SYSTEM_GUID;
 
 static inline int is_bootable(gpt_entry *p)
 {