leds: da903x: fix the building failure of incomplete type of 'work'

The leds-da903x LED driver was missing the proper #include of
linux/workqueue.h, but happened to compile on ARM due to implied
includes through other header files.

We do need the explict include on other architectures (reported at least
for x86-64).

Reported-tested-and-acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>