Always install icons, and install them under $(datadir).

We always install them because of bug 10737.

We install them under $(datadir) because that's where we should install
our data - if your desktop environment can't find the icons there,
either it's broken (file a bug) or misconfigured (fix the configuration
or, if you didn't configure it, file a bug).

Bug: 10737
Change-Id: I567269d8e45e6543d9e39dbedc49830adf7edb9f
Reviewed-on: https://code.wireshark.org/review/5576
Reviewed-by: Guy Harris <guy@alum.mit.edu>