gtkmm3: depends on libgtk3

gtkmm3: depends on libgtk3

packages shouldn't select libgtk3 directly, just depend on it, like for
libgtk2.
In the past libgtk3 didn't require any *GL backend and the dead-end
solution/last resort was the broadway (networked) gdk backend - though
not very useful it didn't require any funky dependencies.
But now we do. Fixes:
http://autobuild.buildroot.net/results/794/794c7ed221432e46a810fc281732ba417cd4cda3/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>