XFA: Add another valid fxcodec DEPS entry.

TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1591833002 .

core/src/fxcodec/codec/DEPS

diff --git a/core/src/fxcodec/codec/DEPS b/core/src/fxcodec/codec/DEPS
index 9caa4d8..bfbbd82 100644
--- a/core/src/fxcodec/codec/DEPS
+++ b/core/src/fxcodec/codec/DEPS
@@ -1,3 +1,4 @@
 include_rules = [
   '+third_party/libpng/png.h',
+  '+third_party/libtiff/tiffiop.h',
 ]