Update zlib headers to 1.2.7.

The NDK's minimum target is now android-16 which had at least 1.2.7.

Test: make checkbuild
Bug: https://github.com/android-ndk/ndk/issues/230
Change-Id: I4e1e5b3613bdf54d101a99358107c5a52dd09d71
6 files changed