firefox: workaround for freetype-config breakage

All binconfig scripts in OE-core have been disabled, leading to -I/usr/include/freetype2 disappearing from C(PP)FLAGS. Add it in manually as a workaround, mozilla autotools are a maze and not easily fixed.

Signed-off-by: Koen Kooi <koen.kooi@linaro.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>