fix: add build tag to docker benchmark test to match MockWatcherFactory

MockWatcherFactory is defined in add_docker_metadata_test.go which has
//go:build (linux || darwin || windows) && !integration. The benchmark
must use the same constraint or the linter can't resolve the symbol.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: strawgate

libbeat/processors/add_docker_metadata/add_docker_metadata_benchmark_test.go

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//go:build (linux || darwin || windows) && !integration
+
 package add_docker_metadata
 
 import (