Don't do the desktop integration stuff if we're not building Wireshark.

Wireshark is the desktop app; if we're not building it (i.e., we're only
building the command-line TShark), we don't need any desktop files.

Change-Id: Id8244b417fd53c9d55b4d77fe3ad748c98868ad7
Reviewed-on: https://code.wireshark.org/review/5615
Reviewed-by: Guy Harris <guy@alum.mit.edu>