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/tools/eficapsule.h b/tools/eficapsule.h
index 6efd07d..97d0775 100644
--- a/tools/eficapsule.h
+++ b/tools/eficapsule.h
@@ -24,7 +24,7 @@
 
 #define ARRAY_SIZE(x)		(sizeof(x) / sizeof((x)[0]))
 
-typedef struct {
+typedef struct efi_guid {
 	uint8_t b[16];
 } efi_guid_t __aligned(8);