Fixed duplicated symbols

[kstenschke]

Using miniz-cpp from multiple translation units caused linker error.
Done: marked declarations as static for correct linking from multiple translation units.

[yhyu13]

@kstenschke thanks, this is helpful

[wzel]

Greetings from the GNU Guix community. Merging this pull request would help us in packaging docxBox. Guix issue tracker reference here. Thanks for considering the merge.

[tope99]

I think a better approach is to #define MINIZ_HEADER_FILE_ONLY everywhere so only declarations get included. Then in one file include it with that macro #undefed. This is similar to how stb libraries are used. However:

1. For some reason the macro works in the opposite way - in stb you only #define it once before you include the implementation not every time when you include interface. This seems to be more verbose.

2. Last part of zip_file.h is not guarded with this macro so you still get duplicated symbols.

Sadly I can't use miniz-cpp in its current form. It needs to be patched.