build fail, ubuntu 18.04 gcc7.3

[NicoG60]

Here is the error : zip_file.hpp:5611:30: error: invalid conversion from ‘std::size_t ()(void, uint64_t, const void, std::size_t) {aka long unsigned int ()(void, long unsigned int, const void, long unsigned int)}’ to ‘mz_file_writefunc {aka long unsigned int ()(void, long long unsigned int, const void*, long unsigned int)}’ [-fpermissive] archive->m_pWrite = &detail::write_callback;

build and works well on macOS 10.14, clang v10

[iamOgunyinka]

What I did to fix this up is to define mz_uint64 as std::uint64_t in here and I got it to compile.

[NicoG60]

That's exactly what I did in my pull request #7