gitlab-ci: Add a pkg-config for mingw

The one debian provides is broken in buster+, so I've just written my
own. This allows meson to find the installed zlib and prevents it from
falling back to wraps.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 258430d..f67f9bb 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -14,7 +14,7 @@
 # repository's registry will be used there as well.
 variables:
   UPSTREAM_REPO: mesa/mesa
-  DEBIAN_TAG: "2019-10-07_1"
+  DEBIAN_TAG: "2019-10-16"
   DEBIAN_ARM64_TAG: "arm64v8-2019-08-09"
   STRETCH_TAG: "2019-09-18"
   DEBIAN_VERSION: buster-slim