libertas/if_spi: needs linux/semaphore.h

libertas/if_spi.c needs to #include linux/semaphore.h to fix build errors:

drivers/net/wireless/libertas/if_spi.c:59: error: field 'spi_ready' has incomplete type
drivers/net/wireless/libertas/if_spi.c:60: error: field 'spi_thread_terminated' has incomplete type
drivers/net/wireless/libertas/if_spi.c:785: error: implicit declaration of function 'down_interruptible'
drivers/net/wireless/libertas/if_spi.c:787: error: implicit declaration of function 'up'
drivers/net/wireless/libertas/if_spi.c:834: error: implicit declaration of function 'down'
drivers/net/wireless/libertas/if_spi.c:943: error: implicit declaration of function 'sema_init'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>