freetype: bump version to 2.9.1

According to [1]:

- fixes CVE-2018-6942: A NULL pointer dereference in the Ins_GETVARIATION()
  function within ttinterp.c could lead to DoS via a crafted font file

- needs '--enable-freetype-config' for freetype-config installation

[1] https://git.savannah.gnu.org/cgit/freetype/freetype2.git/tree/docs/CHANGES?id=86bc8a95056c97a810986434a3f268cbe67f2902



[Peter: also pass --enable-freetype-config for host variant]
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>