plugin: Use the Freetype developer's proposed way if #include'ing their headers.

Allows us to build against Freetype 2.5.1 (without breaking older
versions).

https://bugzilla.gnome.org/show_bug.cgi?id=720573

Signed-off-by: Hans Breuer <hans@breuer.org>
(cherry picked from commit 0b863adcad031c70601e375d72dae64fdd967752)