mark geo-scheme-handler as non-gui executable on macOS/Windows

got this error during configuring the project:

```
CMake Error at src/geo-scheme-handler/CMakeLists.txt:12 (install):
  install TARGETS given no BUNDLE DESTINATION for MACOSX_BUNDLE executable
  target "kde-geo-uri-handler".
```

the tool itself works fine (though its usefulness on macOS might be
questionable), I could successfully launch Apple's Maps.app like this:

```
% ./kde-geo-uri-handler --coordinate-template "maps://?ll=<LAT>,<LON>" --query-template "maps://?q=<Q>" 'geo:48.305,14.286'
% ./kde-geo-uri-handler --coordinate-template "maps://?ll=<LAT>,<LON>" --query-template "maps://?q=<Q>" 'geo:?q=plus+city+pasching'
```