GN: Fix missing pkg_config import in GL back-end. This was causing a build break on ChromeOS builers and brekaing the ANGLE roll into Chromium. Bug: chromium:1129075 Bug: angleproject:3943 Change-Id: Ic7976c0b506f5af67823da8de1f8c850e43abee0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2415169 Reviewed-by: Jamie Madill <jmadill@chromium.org>