android: metadata: Add SPDX tag

Add SPDX tag to the android metadata library files licensed under
Apache-2.0.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/android/metadata/camera_metadata.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: Apache-2.0 */
 /*
  * Copyright (C) 2012 The Android Open Source Project
  *

src/android/metadata/camera_metadata_tag_info.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: Apache-2.0 */
 /*
  * Copyright (C) 2012 The Android Open Source Project
  *