cosmetic: guard dead osmux vty code with ifdef

Since currently osmux is not available we decided to lock down the
respective VTY command with an early return CMD_WARNING, making
the code after this line unreachable.

Guard the dead code with an ifdef

Fixes: Coverity CID#178648
Change-Id: I2ad9579453f52fe129cf120801a2efa19a26304e
1 file changed