fix splash screen icon not shown

Currently weather apps splash screen does not show the icon. The error message states, that /usr/share/click/preinstalled/.click/users/@all/weather.ubports/weather-app-splash.svg can not be found. The icon actually does get installed into the share/app subfolder.

I think when doing this commit, only the path for the icon did get changed, but not for the splash screen icon.

This MR should fix that. But since I am not very experienced with cmake stuff, it would be good someone can check that.