Make linking to custom zlib private in libzsync

[lgisler]

I encountered compile errors in
zsync2/src/zsclient.cpp due to lib/zlib/zlib.h being included I'm assuming incorrectly since the building instructions suggest installing zlib1g-dev and zlib/zlib.h is missing the gzwrite function (see zlib/zlib.h:980).

[TheAssassin]

Looks good to me. But we'll have to see if it breaks AppImageUpdate.

I suppose librcksum is also a candidate for private linking?

[lgisler]

The rcksum.h is included in src/zsmake.cpp:12
I was unable to find an apt package that provides rcksum.h so perhaps it is best to leave it public.

[TheAssassin]

I was unable to find an apt package that provides rcksum.h so perhaps it is best to leave it public.

It's included in a source file, but not elsewhere, so marking it PRIVATE will cause CMake to still add the -I to the compilation of the source unit, but not expose it to other targets which target_link_library(<target> ... zsync2). Therefore it shouldn't hurt.

[lgisler]

Making the linking of librcksum PRIVATE required adding librcksum to ZSYNC_DEPS explicitly.
I have attached a git diff of the change. If this is preferred I can amend the commit in this PR.

private_librcksum.diff.txt

[TheAssassin]

No, it's fine as-is. Thanks!