Added new symlinks for Whatsapp client 'whatsapp-desktop'

[boatware]

whatsapp-dektop uses Icon=whatsapp in its .desktop file, so I've added new symlinks, named them 'whatsapp.svg' and linked them to the original icons.

I know, I also could have just changed the entry of the .desktop file but for those who don't want to change anything at that place, I think this is a good solution.