USB: fix usb-serial/generic build warning

Fix annoying build warning when CONFIG_USB_SERIAL_GENERIC is undefined.

  drivers/usb/serial/generic.c:24: warning: `generic_probe' declared `static' but never defined
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/generic.c

@@ -20,13 +20,14 @@
 #include <linux/usb/serial.h>
 #include <asm/uaccess.h>
 
-static int generic_probe(struct usb_interface *interface,
-			 const struct usb_device_id *id);
-
 
 static int debug;
 
 #ifdef CONFIG_USB_SERIAL_GENERIC
+
+static int generic_probe(struct usb_interface *interface,
+			 const struct usb_device_id *id);
+
 static __u16 vendor  = 0x05f9;
 static __u16 product = 0xffff;