fontconfig: Force use of c++ linker

This is really silly, but gyp doesn't seem to track if a static_library
contains C++ code and needs a C++ link step. So our copy of libxml
breaks us by containing a little bit of (chromium-added) C++ code. This
is really a correctness issue in the build system.

BUG=445627
TEST=linux_ecs_ozone tryjob

Review URL: https://codereview.chromium.org/872493004

Cr-Commit-Position: refs/heads/master@{#315138}

third_party/fontconfig/fontconfig.gyp

@@ -26,6 +26,7 @@
         }],
       ],
       'sources': [
+        'chromium/empty.cc',
         'src/src/fcarch.h',
         'src/src/fcatomic.c',
         'src/src/fcblanks.c',