mfd: Fix ab3100-otp build failure

ab3100.h should include linux/workqueue.h for otp to build properly.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
diff --git a/include/linux/mfd/ab3100.h b/include/linux/mfd/ab3100.h
index 56343b8..3ead5cf 100644
--- a/include/linux/mfd/ab3100.h
+++ b/include/linux/mfd/ab3100.h
@@ -6,6 +6,7 @@
  */
 
 #include <linux/device.h>
+#include <linux/workqueue.h>
 
 #ifndef MFD_AB3100_H
 #define MFD_AB3100_H