build: Also try finding libxml2 with CMake

Hi,

This adds a change to meson.build to try finding libxml2 using CMake if we do not find it via pkg-config, before we resort to the fallback mechanism (which, builds a pretty old version of libxml2--efforts are underway in libxml++[2]). We do not yet require Meson 0.60.0 or later, so we can't yet combine the libxml2 package strings to search in one shot1.

Sorry, I should have posted this for the master branch, but it's applicable for the gtksourceview-5-4 branch as well.

With blessings, thank you!