GSK has various enumeration types that are currently not used; while
they may go away, currently they are built and introspected. If we want
the introspection machinery to work, and still use static libraries to
build GDK and GSK into the GTK shared library, then we need to reference
the get_type() function of these enumeration types somewhere, to avoid
the linker discarding it, and thus breaking the build.

As luck would have it, we have an autogenerated bit of C that refers to
all the get_type() functions in the library; if we add the GSK types to
it, then we get the reference we're looking for, and the build succeeds.