openjdk10: gtk2 -> gtk3

http://openjdk.java.net/jeps/283 "Enable GTK 3 on Linux" was included
in OpenJDK 9.

nothing else currently in nixpkgs is using 10, so this just lets us
establish a good baseline as things are ported onto it.  if needed,
the build could be parameterized so that any packages that turn out to
need gtk2 could still use it.