Fix libpng build for ARM.

TBR=msarett@google.com

Review-Url: https://codereview.chromium.org/2214543003
diff --git a/third_party/BUILD.gn b/third_party/BUILD.gn
index 9a80d5f..a3878e8 100644
--- a/third_party/BUILD.gn
+++ b/third_party/BUILD.gn
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/arm.gni")
 import("//build_overrides/build.gni")
 import("../pdfium.gni")