gitlab-ci: add the vulkan overlay layer to the vulkan build

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index da58cb5..fa82a5e 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -222,6 +222,8 @@
       -D gallium-opencl=disabled
     VULKAN_DRIVERS: intel,amd,freedreno
     LLVM_VERSION: "7"
+    EXTRA_OPTION: >
+      -D vulkan-overlay-layer=true
 
 scons-nollvm:
   extends: .scons-build