build: install a login manager session entry

This makes it possible to run Weston session from GDM (and possibly
other login managers).
diff --git a/src/weston.desktop b/src/weston.desktop
new file mode 100644
index 0000000..102ee17
--- /dev/null
+++ b/src/weston.desktop
@@ -0,0 +1,5 @@
+[Desktop Entry]
+Name=Weston
+Comment=The reference Wayland server
+Exec=dbus-launch --exit-with-session weston
+Type=Application