Ensure that GTK is linked on linux

[silasary]

Ran into this while trying to get MSE building on Ubuntu.

For whatever reason, when building against wxWidgets 3.1, it didn't automatically pull in GTK+, so I had to manually add it to the linker.

[twanvl]

It shouldn't be necessary to manually link to GTK. Cmake should add this automatically.
That said, I am using the latest stable release of wxGTK (3.0.5), because that is what my package manager provides. So this might be broken in the development version wxGTK 3.1.