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

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).

Signed-off-by: Simon McVittie <smcv@debian.org>