correct tautological comparison for key code mapping

This fixes two warnings on OS X with clang:

```
cvt/cvt/gui/internal/OSX/OSXKeyMap.h:186:16: warning: comparison of constant 'KEY_Nobreakspace' (160) with expression of type 'char' is always false [-Wtautological-constant-out-of-range-compare]
                                ( str[ 0 ] >= KEY_Nobreakspace && str[ 0 ] <= KEY_ydiaeresis ) )
                                  ~~~~~~~~ ^  ~~~~~~~~~~~~~~~~
cvt/cvt/gui/internal/OSX/OSXKeyMap.h:186:48: warning: comparison of constant 'KEY_ydiaeresis' (255) with expression of type 'char' is always true [-Wtautological-constant-out-of-range-compare]
                                ( str[ 0 ] >= KEY_Nobreakspace && str[ 0 ] <= KEY_ydiaeresis ) )
```

I think the implications are clear.