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: NJason Wessel <jason.wessel@windriver.com>
CC: Jesse Barnes <jbarnes@virtuousgeek.org>
Reported-by: NRandy Dunlap <randy.dunlap@oracle.com>

include/linux/console.h

@@ -79,8 +79,13 @@ int register_con_driver(const struct consw *csw, int first, int last);
 int unregister_con_driver(const struct consw *csw);
 int take_over_console(const struct consw *sw, int first, int last, int deflt);
 void give_up_console(const struct consw *sw);
+#ifdef CONFIG_HW_CONSOLE
 int con_debug_enter(struct vc_data *vc);
 int con_debug_leave(void);
+#else
+#define con_debug_enter(vc) (0)
+#define con_debug_leave() (0)
+#endif
 
 /* scroll */
 #define SM_UP       (1)