ESP32 IDF v4.2 compile fix

[CalebProvost]

Using char for c_in limits it to 8-bit boundaries. Updated to unsigned int for 32-bit processes.

ESP32 compile error resolved:

unishox2.c:576:46: error: array subscript has type 'char' [-Werror=char-subscripts]
         ol = append_code(out, ol, usx_code_94[c_in], &state, usx_hcodes, usx_hcode_lens);

[siara-cc]

Hi, Thank you for the PR. I am still making some improvements to unishox2.c. So I will take care of your problem while I am at it. I am thinking of fixing at the point where the compiler throws error using a typecast instead of changing the datatype of c_in to favor 8-bit machines out there.

[siara-cc]

@CalebProvost This has been fixed now