[WebPayments] Adding missing include to mojom BUILD

This is a continuation for:
https://chromium-review.googlesource.com/c/chromium/src/+/848085

There was a missing include which only became apparent when
attempting to build downstream. I have done more extensive local
testing and believe this should resolve the matter.

Bug: 795007
Change-Id: I368f14d00e784082f887f889f884b992daf06b86
Reviewed-on: https://chromium-review.googlesource.com/851153
Commit-Queue: Tommy Martino <tmartino@chromium.org>
Commit-Queue: Mathieu Perreault <mathp@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527130}

components/payments/mojom/BUILD.gn

@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/android/rules.gni")
+
 java_group("mojom_java") {
   # This is a temporary empty target. It is needed to enable prerequisite
   # downstream changes to be landed without breaking the Android build. It will