kbuild: uapi: fail header test on compiler warnings

Compiler warnings also indicate issues with the headers.

Make sure they don't go unnoticed.

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-v2-2-8a7921ca3a03@linutronix.de
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: t-8ch

usr/include/Makefile

@@ -80,7 +80,7 @@ always-y := $(patsubst $(obj)/%.h,%.hdrtest, $(shell find $(obj) -name '*.h' 2>/
 # Include the header twice to detect missing include guard.
 quiet_cmd_hdrtest = HDRTEST $<
       cmd_hdrtest = \
-		$(CC) $(c_flags) -fsyntax-only -x c /dev/null \
+		$(CC) $(c_flags) -fsyntax-only -Werror -x c /dev/null \
 			$(if $(filter-out $(no-header-test), $*.h), -include $< -include $<); \
 		$(PERL) $(src)/headers_check.pl $(obj) $<; \
 		touch $@