Staging: comedi: serial2002: fix include build issue

Now that sched.h was removed from poll.h, serial2002.c needs
to include it otherwise it does not build properly.

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/comedi/drivers/serial2002.c

@@ -35,6 +35,7 @@ Status: in development
 
 #include <linux/delay.h>
 #include <linux/ioport.h>
+#include <linux/sched.h>
 
 #include <asm/termios.h>
 #include <asm/ioctls.h>