meson: Fix building as a subproject
meson: Fix building as a subproject

meson.source_root() returns the toplevel source directory
of the toplevel project, thus the paths were wrong when using
it. Simply using files() gets us the right path

https://bugzilla.gnome.org/show_bug.cgi?id=784133

johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib