samples: synchronization: build this sample on all platforms

Try to catch issues related to platforms and HAL changes using this
basic sample. We have the same flag for a kernel test, but this is
skipped if no kernel changes are present in the PR, so we were missing
some coverage of platform related changes.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

samples/synchronization/sample.yaml

@@ -5,6 +5,7 @@ sample:
 tests:
   sample.kernel.synchronization:
     tags: synchronization
+    build_on_all: true
     harness: console
     harness_config:
       type: multi_line