transmission: gtk option needs libgtk3

Fixes the following configure issue:

checking for GTK... no
configure: error: Package requirements (gtk+-3.0 >= 3.4.0
                              glib-2.0 >= 2.32.0
                              gio-2.0 >= 2.26.0,
                              gmodule-2.0 >= 2.32.0
                              gthread-2.0 >= 2.32.0) were not met:

libgtk2 support was dropped in commit cdd71c642724 ((trunk gtk) #4970 remove
deprecated GTK+ API calls, raise GTK+ dependency to 3.2) which was part of
transmission-2.61.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>