gio: Provide schemas directory information in pkg-config file
GSettings XML schema files are installed in a well known directory under Glib's installation directory:
glib-2.0/schemas. However, the Glib installation directory might vary, so the exact location of the schema files might be unknown.
The information regarding this directory has been added to GIO's pkg-config file, so it can be checked, and also overrided, by using the command line utility.