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 <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/driver-api/usb/usb.rst

···
 .. kernel-doc:: drivers/usb/core/urb.c
    :export:
 
+.. c:namespace:: usb_core
 .. kernel-doc:: drivers/usb/core/message.c
    :export: