usb: fix serial build when SYSRQ is disabled

Fix build error when CONFIG_MAGIC_SYSRQ is not enabled:

drivers/usb/serial/generic.c:566: error: implicit declaration of function 'handle_sysrq'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/generic.c

@@ -12,6 +12,7 @@
 #include <linux/kernel.h>
 #include <linux/errno.h>
 #include <linux/slab.h>
+#include <linux/sysrq.h>
 #include <linux/tty.h>
 #include <linux/tty_flip.h>
 #include <linux/module.h>
@@ -558,6 +559,7 @@ void usb_serial_generic_unthrottle(struct tty_struct *tty)
 	}
 }
 
+#ifdef CONFIG_MAGIC_SYSRQ
 int usb_serial_handle_sysrq_char(struct tty_struct *tty,
 			struct usb_serial_port *port, unsigned int ch)
 {
@@ -571,6 +573,13 @@ int usb_serial_handle_sysrq_char(struct tty_struct *tty,
 	}
 	return 0;
 }
+#else
+int usb_serial_handle_sysrq_char(struct tty_struct *tty,
+			struct usb_serial_port *port, unsigned int ch)
+{
+	return 0;
+}
+#endif
 EXPORT_SYMBOL_GPL(usb_serial_handle_sysrq_char);
 
 int usb_serial_handle_break(struct usb_serial_port *port)