usb: fix serial build when SYSRQ is disabled

Fix build error when CONFIG_MAGIC_SYSRQ is not enabled:

drivers/usb/serial/generic.c:566: error: implicit declaration of function 'handle_sysrq'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>