XFA: Add missing libtiff file from commit 9c2c87d1c744.

TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1571523002 .
diff --git a/third_party/BUILD.gn b/third_party/BUILD.gn
index ed8037b..0ad92ab 100644
--- a/third_party/BUILD.gn
+++ b/third_party/BUILD.gn
@@ -306,6 +306,7 @@
     ]
     sources = [
       "libtiff/tiffiop.h",
+      "libtiff/tiffvers.h",
       "libtiff/tif_aux.c",
       "libtiff/tif_close.c",
       "libtiff/tif_codec.c",