um: Prevent building modules incompatible with MODVERSIONS

The manual ld invocation in arch/um/drivers doesn't play nicely with
genksyms. Given the problematic modules are deprecated anyway, just
prevent building them when using MODVERSIONS.

e.g.
MODPOST Module.symvers
arch/um/drivers/.pcap.o.cmd: No such file or directory

Signed-off-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: Richard Weinberger <richard@nod.at>