coccinelle: api/atomic_as_refcounter: include message type in output

A common practice is to grep for "WARNING" or "ERROR" text in the report
output from a Coccinelle semantic patch script. So, include the text
"WARNING: " in the report output generated by the semantic patch for
desired filtering of the output. Also improves the readability of the
output. Here is an example of the old and new outputs reported:

    xyz_file.c:131:39-40: atomic_add_unless
    xyz_file.c:131:39-40: WARNING: atomic_add_unless

    xyz_file.c:196:6-25: atomic_dec_and_test variation before object free at line 208.
    xyz_file.c:196:6-25: WARNING: atomic_dec_and_test variation before object free at line 208.

Signed-off-by: Deepak R Varma <drv@mailo.com>
Acked-by: Julia Lawall <Julia.Lawall@inria.fr>