[zlib] Don't typedef `ptrdiff_t`.

While using zlib in 'solo' mode (via the `Z_SOLO` macro), we actually
include some standard header files, making the typedef fail on systems where
the native `ptrdiff_t` type differs.

Fixes #1124.

* src/zlib/zutil.h: Comment out definition; it doesn't work on Windows.

* src/zlib/patches/freetype-zlib.diff: Updated.
2 files changed