depend on fontconfig for linux rather than x11

https://codereview.chromium.org/99103006 moved WebFontInfo.cpp from
core to platform.  In core fontconfig is enabled if OS=="linux",
but in platform it is enabled if use_x11==1.  In most setups this
works because the headers are in the same base directory, but if
that's not the case the build will fail.

To fix this, I think platform should just depend on fontconfig if
OS=="linux".

BUG=303332

Review URL: https://codereview.chromium.org/122653004

git-svn-id: svn://svn.chromium.org/blink/trunk@164428 bbb929c8-8fbe-4397-9dbb-9b2b20218538