gtk/gtkfontchooserwidget.c: Include FreeType Headers properly

Like what was done in the master branch, we ought to include the
FreeType headers via the macros that are defined via including ft2build.h.
2 jobs for ft.headers in 59 minutes and 43 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #181686
build

00:07:00

passed #181687
win32 allowed to fail manual
msys2-mingw32

00:52:42