debian: fix lintian warning - command-with-path-in-maintainer-script

Reported by @adrelanos
https://github.com/QubesOS/qubes-core-agent-linux/pull/39#issuecomment-280951206

debian/qubes-core-agent.postinst

@@ -141,7 +141,7 @@ case "${1}" in
             dpkg-statoverride --update --add user user 775 /var/lib/qubes/dom0-updates
         fi
 
-        /usr/bin/glib-compile-schemas /usr/share/glib-2.0/schemas
+        glib-compile-schemas /usr/share/glib-2.0/schemas
 
 
         # Update Qubes App Menus