scripts/checkpatch: Fix a typo

When running checkpatch.pl on a commit adding a file without
SPDX tag we get:

  Undefined subroutine &main::WARNING called at ./scripts/checkpatch.pl line 1694.

The WARNING level is reported by the WARN() method. Fix the typo.

Fixes: fa4d79c64da ("scripts: mandate that new files have SPDX-License-Identifier")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20250303172508.93234-1-philmd@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

scripts/checkpatch.pl

@@ -1691,8 +1691,8 @@ sub process {
 				  "'SPDX-License-Identifer'");
 			} else {
 			    # Other files MAY have SPDX license if appropriate
-			    WARNING("Does new file '$expect_spdx_file' need " .
-				    "'SPDX-License-Identifer'?");
+			    WARN("Does new file '$expect_spdx_file' need " .
+				 "'SPDX-License-Identifer'?");
 			}
 		    }
 		    $expect_spdx = 1;