Unhook the Link Instruments MSO-19 from the build for now.

Unhook the Link Instruments MSO-19 from the build for now.

This driver is neither working nor has it been in a compiling state for
a long time, so unhook it from the build until it is fixed and works.

The files (api.c and protocol.[ch]) are still in git, but won't end up in
released tarballs and they don't get built (neither git nor tarballs).

This also allows us to drop the otherwise unneeded dependency on libudev.
When the MSO-19 driver comes back, it should be in a form that doesn't
require the inherently Linux-only libudev anyway. See also:
http://sigrok.org/bugzilla/show_bug.cgi?id=65