console: Fix compilation regression

A regression of building without CONFIG_HW_CONSOLE was introduced with
commit b45cfba4

 (vt,console,kdb:
implement atomic console enter/leave functions).

ERROR: "con_debug_enter" [drivers/serial/kgdboc.ko] undefined!
ERROR: "vc_cons" [drivers/serial/kgdboc.ko] undefined!
ERROR: "fg_console" [drivers/serial/kgdboc.ko] undefined!
ERROR: "con_debug_leave" [drivers/serial/kgdboc.ko] undefined!

When there is no HW console the con_debug_enter and con_debug_leave
functions should have no code.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
CC: Jesse Barnes <jbarnes@virtuousgeek.org>
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>