Added source code of zlib-1.2.8

The ZLIB_VERSION, ZLIB_VERNUM and ZLIB_VER_SUBREVISION have been
changed to comply with zlib license requirement.

* src/zlib/deflate.c: Changed version to 1.2.8.f-git2r

* src/zlib/inftrees.c: Changed version to 1.2.8.f-git2r

* src/zlib/zlib.h: Changed version to 1.2.8.f-git2r