Generate BUILD files for grpc-gateway by default (bazel-contrib#1578)

Specify that Gazelle should generate BUILD files for the
"github.com/grpc-ecosystem/grpc-gateway/v2" Go module, since the BUILD
files already present in that module's source repository contain
references to Bazel repositories that are no longer created
automatically by Gazelle.

internal/bzlmod/default_gazelle_overrides.bzl

@@ -3,6 +3,7 @@ visibility("private")
 DEFAULT_BUILD_FILE_GENERATION_BY_PATH = {
     "github.com/envoyproxy/protoc-gen-validate": "on",
     "github.com/google/safetext": "on",
+    "github.com/grpc-ecosystem/grpc-gateway/v2": "on",
 }
 
 DEFAULT_DIRECTIVES_BY_PATH = {