GL_EXT_disjoint_timer_query was missing the "GL_".

Bug: b/171446929
Change-Id: I6e11f18bfa3da18efc65691a94a6290fbcf16777
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2502309Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>