gtksourceview/meson.build: Fix generating pkg-config
gtksourceview/meson.build: Fix generating pkg-config

libxml2 could be found either via pkg-config or via manually looking for
headers and libraries, so make sure we generate the pkg-config file
appropriately.

GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview