Remove unused includes and forward declarations in libmediaextractor/include

Test: make checkbuild
Bug: 67908556
Change-Id: If840305af5c043bb8014590eb208cf11c99e61ee
diff --git a/media/libmedia/nuplayer2/GenericSource.h b/media/libmedia/nuplayer2/GenericSource.h
index 0666d27..5a71edb 100644
--- a/media/libmedia/nuplayer2/GenericSource.h
+++ b/media/libmedia/nuplayer2/GenericSource.h
@@ -33,6 +33,7 @@
 struct ARTSPController;
 class DataSource;
 class IDataSource;
+class IMediaSource;
 struct MediaHTTPService;
 struct MediaSource;
 class MediaBuffer;