usb: Restore the usb_header label

Commit caa93d9b ("usb: Fix up movement of USB core kerneldoc location")
removed the reference to the _usb_header label by mistake, which causes the
following htmldocs build warning:

Documentation/driver-api/usb/writing_usb_driver.rst:129: WARNING: undefined label: usb_header

Restore the label.

Fixes: caa93d9b ("usb: Fix up movement of USB core kerneldoc location")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20210521013608.17957-1-festevam@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/usb/usb.rst

@@ -123,6 +123,8 @@ are in ``drivers/usb/common/common.c``.
 In addition, some functions useful for creating debugging output are
 defined in ``drivers/usb/common/debug.c``.
 
+.. _usb_header:
+
 Host-Side Data Types and Macros
 ===============================