Xcode / iOS support

I'm attempting to get your minizip library working on iOS.

The first issues I came across is the include paths.

```
#include "aes\\aes.h"
```

If this breaks compatibility with Windows I can wrap the include in a #ifdef

Secondly zip.c fails to compile with #define HAVE_AES due to entropy_fun being windows only.
For the moment I have undef HAVE_AES if _WIN32 is not available.

We these commits the Library compiles and works on iOS. Unzipping AES files is supported but Zipping is not supported until I can replace entropy_fun. 

Thanks for the great library