pwm-gpio-custom: fix compile on linux 2.6.31

SVN-Revision: 22721

package/pwm-gpio-custom/src/pwm-gpio-custom.c

@@ -39,6 +39,7 @@
 #include <linux/module.h>
 #include <linux/init.h>
 #include <linux/platform_device.h>
+#include <linux/workqueue.h>
 
 #include <linux/pwm/pwm.h>