A few years ago, libfreetype introduced FT_Get_Font_Format() as an alias for
FT_Get_X11_Font_Format(), but FT_Get_X11_Font_Format() was kept for abi
backwards-compatibility.
Our qt bump to 5.9 introduced a call to FT_Get_Font_Format(). Replace it with
FT_Get_X11_Font_Format() in order to remain compatibile with older freetype,
which is still used by e.g. Ubuntu Trusty.