[BEAM-5379] Ignore go.sum files in RAT checks

R: @youngoli 
cc: @boyuanzz @lukecwik @damondouglas

build.gradle

@@ -74,6 +74,9 @@ rat {
     "**/apache_beam/portability/api/*_pb2*.py",
     "**/go/pkg/beam/**/*.pb.go",
 
+    // Ignore go.sum files, which don't permit headers
+    "**/go.sum",
+
     // Ignore Go test data files
     "**/go/data/**",