#i124394# fix inclusion of freetype's ftsynth.h header

the location of this header file changed with freetype 2.51 so we had to adapt.
On the other hand the header is not actually needed because we load the symbols
for synthetization of outlines dynamically to be compatible with freetype libs
that don't support them. So we can remove the header inclusion.

Found-by: Ariel Costenla-Haile <ariel@apache.org>
Patch-by: Herbert Duerr <hdu@apache.org>