.github/zephyr: add -Werror

Because we can. This would have caught this regression from
commit 287a5f9 ("ssp: mclk/bclk turned off unexpectedly")

-DEXTRA_CFLAGS is not very well documented but it is what Zephyr uses,
try `git -C zephyr grep -C 5 EXTRA_CFLAGS and see.

```
sof/src/drivers/intel/ssp/ssp.c: In function 'ssp_set_config_tplg':
sof/zephyr/include/sof/trace/trace.h:44:11: warning:
                   too many arguments for format [-Wformat-extra-args]
   44 |    printk("%llu: " format "\n", platform_timer_get(NULL), \
      |           ^~~~~~~~
  ...
sof/src/drivers/intel/ssp/ssp.c:763:4: note: in expansion of macro 'dai_info'
  763 |    dai_info(dai, "ssp_set_config(): hw_free stage:
                     ignore since there is still user", dai->index);
```

Using -Werror only in CI avoids slowing down developers with temporary
warnings they intend on fixing later (but before submission)

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

.github/workflows/zephyr.yml

@@ -19,4 +19,4 @@ jobs:
         run: docker run -v "$(pwd)":/workdir
              -e ZEPHYR_SDK_INSTALL_DIR='/opt/toolchains/zephyr-sdk-0.13.0'
              docker.io/zephyrprojectrtos/zephyr-build:v0.18.3
-             ./zephyr/docker-build.sh
+             ./zephyr/docker-build.sh -- -DEXTRA_CFLAGS='-Werror'