fix compiling with FreeType 2.5.1

Excerpt from FT 2.5.1 change log:

> - The header  file layout  has been changed.   After installation,
>       all files are now located in `<prefix>/include/freetype2'.
> 
>       Applications  that  use   (a)  `freetype-config'  or  FreeType's
>       `pkg-config' file to get the include directory for the compiler,
>       and (b) the documented way for header inclusion like
> 
>         #include <ft2build.h>
>         #include FT_FREETYPE_H
>         ...
> 
>       don’t need any change to the source code.