kdb: Allow kernel loadable modules to add kdb shell functions

In order to allow kernel modules to dynamically add a command to the
kdb shell the kdb_register, kdb_register_repeat, kdb_unregister, and
kdb_printf need to be exported as GPL symbols.

Any kernel module that adds a dynamic kdb shell function should only
need to include linux/kdb.h.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>