Fix Freetype dependencies.

Error message:
```
[ 19%] Building CXX object src/libOpenImageIO/CMakeFiles/OpenImageIO.dir/imagebufalgo.cpp.o
In file included from /home/mariusz/myrepos/oiio/src/libOpenImageIO/imagebufalgo.cpp:55:0:
/usr/include/ft2build.h:56:38: fatal error: freetype/config/ftheader.h: No such file or directory
 #include <freetype/config/ftheader.h>
                                      ^
compilation terminated.
src/libOpenImageIO/CMakeFiles/OpenImageIO.dir/build.make:287: recipe for target 'src/libOpenImageIO/CMakeFiles/OpenImageIO.dir/imagebufalgo.cpp.o' failed
```