Sync fx_tiff target conditional with its dependent's.

The dependent for fx_tiff is gated by pdf_enable_xfa_tiff, so make the
target itself match.

Change-Id: I829977d3b470c5c5388f4069e650abc0b57d5c6c
Reviewed-on: https://pdfium-review.googlesource.com/c/43892
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

third_party/BUILD.gn

diff --git a/third_party/BUILD.gn b/third_party/BUILD.gn
index 0430832..674a847 100644
--- a/third_party/BUILD.gn
+++ b/third_party/BUILD.gn
@@ -479,7 +479,7 @@
   }
 }
 
-if (pdf_enable_xfa) {
+if (pdf_enable_xfa_tiff) {
   source_set("fx_tiff") {
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [