gtk4.pc: Move third-party libraries from Libs to Libs.private

This avoids "overlinking". These libraries are an implementation detail of particular backends, so they only need to appear on library consumers' linker command-lines if the dependency cannot be picked up from the shared library automatically (when linking statically, or when building on a deficient OS that doesn't support transitive dependencies between shared libraries, in which case pkg-config should be built with --enable-indirect-deps).