Added usb_string function to a namespace

Added usb_string function to a namespace

with reference to WARNING:
Duplicate C declaration, also defined at driver-api/usb/gadget:804
There is a function usb_string in the file message.c,
there is also a struct usb_string in the kernel api headers.
The docs is unable to index the function as the index is occupied by struct
This fix adds messgae.c to the usb_core namespace (in docs) hence providing
usb_sting a unique index usb_core.usb_string()

Signed-off-by: Kevin Paul Reddy Janagari <kevinpaul468@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250405165116.147958-1-kevinpaul468@gmail.com