avformat/file: Constify android content protocol

(The discrepancy between the definition and the declaration
in protocols.c is actually UB.)

Reviewed-by: Matthieu Bouron <matthieu.bouron@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/file.c

@@ -670,7 +670,7 @@ done:
     return ret;
 }
 
-URLProtocol ff_android_content_protocol = {
+const URLProtocol ff_android_content_protocol = {
     .name                = "content",
     .url_open            = android_content_open,
     .url_read            = file_read,