libertas: fix build

drivers/net/wireless/libertas/cmdresp.c: In function ‘lbs_process_event’:
drivers/net/wireless/libertas/cmdresp.c:519: error: ‘TASK_INTERRUPTIBLE’
undeclared (first use in this function)
drivers/net/wireless/libertas/cmdresp.c:519: error: (Each undeclared
identifier is reported only once
drivers/net/wireless/libertas/cmdresp.c:519: error: for each function it
appears in.)

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>