libglnx: Use git.gnome.org's copy

Not the master from my laptop.

diff --git a/libglnx b/libglnx
index 20b507de5c14df8da678d48f9028fe61340c390d..1288bd850829c66935d3f12c3363a32c87c1421c 160000 (submodule)
--- a/libglnx
+++ b/libglnx
@@ -1 +1 @@
-Subproject commit 20b507de5c14df8da678d48f9028fe61340c390d
+Subproject commit 1288bd850829c66935d3f12c3363a32c87c1421c