Staging: solo: add delay.h header

Several solo drivers need <linux/delay.h>, so add it to the
private solo6010.h header file.

drivers/staging/solo6x10/solo6010-core.c:191: error: implicit declaration of function 'mdelay'
drivers/staging/solo6x10/solo6010-tw28.c:181: error: implicit declaration of function 'msleep_interruptible'
drivers/staging/solo6x10/solo6010-gpio.c:78: error: implicit declaration of function 'udelay'
drivers/staging/solo6x10/solo6010-v4l2-enc.c:254: error: implicit declaration of function 'mdelay'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>