spi: fix compile error

Fix compile error below:

     LD      drivers/spi/built-in.o
     CC [M]  drivers/spi/spi_gpio.o
   In file included from drivers/spi/spi_gpio.c:26:
   include/linux/spi/spi_bitbang.h:23: error: field `work' has incomplete type
   make[2]: *** [drivers/spi/spi_gpio.o] Error 1
   make[1]: *** [drivers/spi] Error 2
   make: *** [drivers] Error 2

Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/spi/spi_bitbang.h

@@ -18,6 +18,9 @@
  *	duplex (MicroWire) controllers.  Provide chipslect() and txrx_bufs(),
  *	and custom setup()/cleanup() methods.
  */
+
+#include <linux/workqueue.h>
+
 struct spi_bitbang {
 	struct workqueue_struct	*workqueue;
 	struct work_struct	work;