freetype: fixup config file

Fix freetype-config's includedir and libdir since they're expanded from
configure values since bumping to version 2.4.12

Fixes:
http://autobuild.buildroot.net/results/1c7/1c70ce0ff350623cc38baf4b8b61c8a7a8586aba/


...and many others.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>