Update fontconfig submodule for https://github.com/mozilla-servo/fontconfig/pull/1

This of course depends on https://github.com/mozilla-servo/fontconfig/pull/1 being merged.
auto merge of #2040 : SimonSapin/servo/fontconfig-compile, r=jdm

This of course depends on https://github.com/mozilla-servo/fontconfig/pull/1 being merged.