Fix freetype cmake module

On fedora, the freetype cmake module would not find the correct
location.  This is because it wasn't using the pkg-cofig include
directory, which is different from the main ft2build.h header directory
(which is /usr/include).

This modification may be a good thing to do on all the cmake packages,
actually.  Currently, they all just search for the location of a
specific header, and ignore the pkg-config includes if they find it in
specific hint locations.