youtube: fix assignment compiler warnings
youtube: fix assignment compiler warnings

grl-youtube.c:745:47: warning: passing argument 1 of
gdata_youtube_service_get_categories_async’ from incompatible pointer
  gdata_youtube_service_get_categories_async (service, NULL,
                                              ^~~~~~~
grl-youtube.c:31: expected ‘GDataYouTubeService * but argument
is of type ‘GDataService *
  void gdata_youtube_service_get_categories_async (GDataYouTubeService
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

grl-youtube.c: In function ‘produce_from_feed’:
grl-youtube.c:1076:9: warning: assignment from incompatible pointer
  query = gdata_youtube_query_new (NULL);
  ^

grl-youtube.c: In function ‘produce_from_category’:
grl-youtube.c:1128:9: warning: assignment from incompatible pointer
  query = gdata_youtube_query_new (NULL);
  ^

grl-youtube.c: In function ‘grl_youtube_source_search’:
grl-youtube.c:1317:9: warning: assignment from incompatible pointer
  query = gdata_youtube_query_new (ss->text);
  ^

https://bugzilla.gnome.org/show_bug.cgi?id=770806

grilofw/grilo-plugins,jasuarez/grilo-plugins,MikePetullo/grilo-plugins,GNOME/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,GNOME/grilo-plugins,MikePetullo/grilo-plugins,grilofw/grilo-plugins