scpi_vxi: Unbreak MacOS build (vxi.h inclusion order).

Undo the #include statement order change of commit 7414fb55cb1f. It's
essential on MacOS to include vxi.h early to keep related u_long et al
data types available. See commit d40b8557a962 for a discussion.

Also drop the inclusion of rpc/rpc.h from scpi_vxi.c because it is not
used in that source file, instead is a vxi.h dependency and is included
there.

[ gsi: rephrase commit message ]