uml: make three functions static

Make the following three functions static, since they don't need to be global.

arch/um/drivers/mcast_kern.c::mcast_setup()
arch/um/drivers/mconsole_user.c::mconsole_reply_v0()
arch/um/drivers/port_user.c::port_pre_exec()

Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: WANG Cong <wangcong@zeuux.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>