Fix unused setter argument warning

[msanders]

Fixes:

Sources/HotKey/KeyCombo.swift:26:22: warning: setter argument 'newValue' was never used, but the property was accessed
                        carbonModifiers = modifiers.carbonFlags
                                          ^
Sources/HotKey/KeyCombo.swift:26:22: note: did you mean to use 'newValue' instead of accessing the property's current value?
                        carbonModifiers = modifiers.carbonFlags
                                          ^~~~~~~~~
                                          newValue

[wlisac]

I'm seeing the same warning. Would love to see this make it in.

[puliaiev]

I was about to create a PR and found yours 😁.