Fix compilation warnings in contexpr

[joseluismarti]

Description

Small fix, I've seen it when compiling:

/home/user/inputstream.adaptive/src/utils/Utils.h:41:32: 
warning: ISO C++11 does not allow conversion from string literal to 'char *const' [-Wwritable-strings]
constexpr char* NAME_UNKNOWN = "unk"; // Kodi codec name for unknown codec
...

Since C++11 the string literal is const char*

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• I have read the Contributing document
• My code follows the Code Guidelines of this project
• My change requires a change to the Wiki documentation
• I have updated the documentation accordingly

[CastagnaIT]

thank you i havent found time to do it