x11/nvidia-driver: prevent dependencies non exiting 32bits compat.

Since pkg 2.x the dependency on shared libraries have been increased,
in particular we now track 32bits compat libraries.

The nvidia-driver on amd64 provides 32bits libraryes which are linked to
libX11.so.6 itself 32bit, which nothing in the ports tree provide,
meaning orphans dependencies, which results in tools analysing the
dependencies to complain about missing dependencies.

Add a knob to tell pkg to ignore those 32bits compat libraries when
extracting which libraries the package should depend on.