staging: wilc1000: linux_wlan_spi: include header

A check using 'sparse' shows warnings in linux_wlan_spi.c:

drivers/staging/wilc1000/linux_wlan_spi.c:43:19: warning: symbol 'wilc_spi_dev' was not declared. Should it be static?
drivers/staging/wilc1000/linux_wlan_spi.c:71:19: warning: symbol 'wilc_bus' was not declared. Should it be static?
drivers/staging/wilc1000/linux_wlan_spi.c:95:5: warning: symbol 'linux_spi_init' was not declared. Should it be static?
drivers/staging/wilc1000/linux_wlan_spi.c:195:5: warning: symbol 'linux_spi_write' was not declared. Should it be static?
drivers/staging/wilc1000/linux_wlan_spi.c:320:5: warning: symbol 'linux_spi_read' was not declared. Should it be static?
drivers/staging/wilc1000/linux_wlan_spi.c:365:5: warning: symbol 'linux_spi_write_read' was not declared. Should it be static?
drivers/staging/wilc1000/linux_wlan_spi.c:402:5: warning: symbol 'linux_spi_set_max_speed' was not declared. Should it be static?

Let's avoid it by including "linux_wlan_spi.h" header.

Signed-off-by: Stanislav Kholmanskikh <kholmanskikh.s.s@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>