gui: add semicolon as trailing to .desktop

```
/builddir/build/BUILDROOT/scanmem-0.14-3.de2653d.fc21.x86_64/usr/share/applications/GameConqueror.desktop: error: value "Development;Utility" for string list key "Categories" in group "Desktop Entry" does not have a semicolon (';') as trailing character
```

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>