textproc/raptor2: update to 2.0.16

Changelog:
 * Fixes CVE-2017-18926 and CVE-2020-25713
 * Multiple Appveyor Windows and CMake build fixes by 0u812. (Note: the
   resulting binaries and libraries were not tested on Windows)
 * Turtle parser now reads input in chunks so can handle huge files.
   Patch by Sebastian Freundt
 * Added a serializer for the mKR language. Patch by Richard H.
   McCullough.
 * Rapper utility now counts triples using longs
 * Several smaller portability fixes for OpenBSD and cross building

See the Raptor2 2.0.16 Release Notes [1] for the full details of the changes.

[1] https://librdf.org/raptor/RELEASE.html#rel2_0_16