kbuild: uapi: upgrade check_declarations() warning to error

No problematic declarations exist anymore.

Make sure it stays this way.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250813-kbuild-hdrtest-fixes-v2-5-8a7921ca3a03@linutronix.de
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: t-8ch

usr/include/headers_check.pl

@@ -74,6 +74,7 @@ sub check_declarations
 		printf STDERR "$filename:$lineno: " .
 			      "userspace cannot reference function or " .
 			      "variable defined in the kernel\n";
+		$ret = 1;
 	}
 }