[PATCH] USB: drivers/usb/core/message.c: make usb_get_string() static

After the removal of usb-midi.c, there's no longer any external user of
usb_get_string().

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>