Try to fix windows build failure.

The "\" might be converted to "\\" in windows and causing copybara failure during conversion.

PiperOrigin-RevId: 340164770
Change-Id: I0def5626427735e60e8bff377e35f3998335ca4f

third_party/systemlibs/grpc.bazel.protobuf.bzl

@@ -128,8 +128,8 @@ def get_plugin_args(
     ]
 
 def _get_staged_proto_file(context, source_file):
-    if source_file.dirname == context.label.package or \
-       is_in_virtual_imports(source_file):
+    if (source_file.dirname == context.label.package or
+        is_in_virtual_imports(source_file)):
         return source_file
     else:
         copied_proto = context.actions.declare_file(source_file.basename)