gitlab-ci: Use Werror for the cross build

We allow the build to fail since we're mostly interested in
the debs but this should alert us when builds throw errors.
Signed-off-by: Guido Günther <guido.gunther@puri.sm>

.gitlab-ci.yml

@@ -19,11 +19,12 @@ before_script:
 build:aarch64:
   <<: *tags
   stage: build
+  allow_failure: true
   variables:
     CROSS_BUILD: "ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-"
   script:
     - make      ${CROSS_BUILD} librem5_defconfig
-    - make -j 4 ${CROSS_BUILD} Image dtbs modules
+    - KCFLAGS="-Werror" make -j 4 ${CROSS_BUILD} Image dtbs modules
     - make -j 4 ${CROSS_BUILD} INSTALL_MOD_PATH=modules/ modules_install
   artifacts:
     paths: