Fixed launcher icon not displayed on Ubuntu

In fbmessenger.desktop, the path to the icon was hardcoded (/usr/share/fbmessenger/icon.png). On my machine, the generated Debian package installed the icon to /usr/local/share/fbmessenger/icon.png (which is default behavior for non-distro packages), so the icon could not be found.

By just using the icon name, the desktop environment looks in the standard locations for app icons, like /usr/share/pixmaps or /usr/local/share/pixmaps.