efi: define struct efi_guid

This let's us forward declare efi_guid_t in the UUID code without
pulling in efi.h

Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
diff --git a/include/efi.h b/include/efi.h
index d5af213..84640cf 100644
--- a/include/efi.h
+++ b/include/efi.h
@@ -74,7 +74,7 @@
  * struct { u32 a; u16; b; u16 c; u8 d[8]; }; which is 4-byte
  * aligned.
  */
-typedef struct {
+typedef struct efi_guid {
 	u8 b[16];
 } efi_guid_t __attribute__((aligned(4)));