ci: Build with -Wnonnull

-Wnonnull is sort of fickle and it's an option a lot of consumers
of glib use.

This commit makes sure it gets used on linux during CI as well, so
we can catch compat problems before they hit our users.

.gitlab-ci.yml

@@ -80,7 +80,7 @@ fedora-x86_64:
   stage: build
   needs: []
   variables:
-    CFLAGS: "-coverage -ftest-coverage -fprofile-arcs"
+    CFLAGS: "-coverage -ftest-coverage -fprofile-arcs -Wnonnull"
   script:
     - meson ${MESON_COMMON_OPTIONS}
             --werror
@@ -128,6 +128,8 @@ debian-stable-x86_64:
   image: $DEBIAN_IMAGE
   stage: build
   needs: []
+  variables:
+    CFLAGS: "-Wnonnull"
   script:
     - meson ${MESON_COMMON_OPTIONS}
             --werror