USB: serial: drop unnecessary sysrq include

There's no need to include sysrq.h in the subsystem header.
Reviewed-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NJohan Hovold <johan@kernel.org>

include/linux/usb/serial.h

@@ -17,7 +17,6 @@
 #include <linux/kref.h>
 #include <linux/mutex.h>
 #include <linux/serial.h>
-#include <linux/sysrq.h>
 #include <linux/kfifo.h>
 
 /* The maximum number of ports one device can grab at once */