Bug 1337667 - Add need-xvfb to stylo build Taskcluster configs. r=dustin

This allows `make check` to pass when run against these builds.

MozReview-Commit-ID: BVs6BPifbfW

--HG--
extra : rebase_source : ebad40260b160744e0d2f9dfc52a74b7e5e5ae0e

taskcluster/ci/build/linux.yml

@@ -328,6 +328,7 @@ linux64-stylo/opt:
         secrets: true
         custom-build-variant-cfg: stylo
         tooltool-downloads: public
+        need-xvfb: true
     run-on-projects: [ 'stylo', 'autoland', 'mozilla-inbound', 'mozilla-central', 'try' ]
 
 linux64-stylo/debug:
@@ -353,6 +354,7 @@ linux64-stylo/debug:
         secrets: true
         custom-build-variant-cfg: stylo-debug
         tooltool-downloads: public
+        need-xvfb: true
     run-on-projects: [ 'stylo', 'autoland', 'mozilla-inbound', 'mozilla-central', 'try' ]
 
 linux64-jsdcov/opt: