cogl: Fix build error when GL_ARB_sync is not defined

Commit 41992757 introduced a change to use CoglContext.glFenceSync
but this method is only available when GL_ARB_sync is defined (as
defined on gl-prototypes/cogl-all-functions.h).

This change fixes that.

GNOME/mutter!1123
5 jobs for !1123 with fix-cogl-build-no-gl-arb-sync in 12 minutes and 51 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #627855
check-commit-log

00:00:36

 
  Build
passed #627856
build-mutter

00:09:08

passed #627857
build-without-native-backend-and-wayland

00:01:22

 
  Test
passed #627859
can-build-gnome-shell

00:02:05

passed #627858
test-mutter

00:03:07