Bazel: Change deprecated single file attribute parameter

single_file attribute is deprecated: [1] and should be replaced with
allow_single_file.

[1] https://docs.bazel.build/versions/master/skylark/lib/attr.html#parameters-3

Change-Id: I72f955ac47764c4eeb1ad65ab1f4b27db590521f

tools/bzl/gwt.bzl

@@ -228,7 +228,7 @@ gwt_binary = rule(
             default = Label("@bazel_tools//tools/zip:zipper"),
             cfg = "host",
             executable = True,
-            single_file = True,
+            allow_single_file = True,
         ),
     },
     outputs = {