nextpnr: disable broken GUI build for now

This didn't work remotely (on a server with Nvidia drivers) _or_ on a
local Intel machine with integrated graphics. I presumably messed
something up (a missing dependency), but I'm not sure where. We can fix
it later.

In the mean time, just disable this by hiding it behind a minimal flag.
As a bonus this reduces the closure size by about half, although it's
still surprisingly large (~300MB or so). Part of that is probably
Python, though.

When the GUI is reintroduced in a working manner, we can expose two
nextpnr attributes for the minimal non-GUI build, and the GUI build.
(Note that I have no plans of making Python optional, since it's
extremely valuable in general and much more lightweight than qtbase.)

Signed-off-by: Austin Seipp <aseipp@pobox.com>