kbuild: respect CONFIG_WERROR for userprogs

The userprogs compiler and linker do not share the regular compiler flags.

Make sure they also fail on warnings with CONFIG_WERROR and W=e.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250814-kbuild-werror-v2-5-c01e596309d2@linutronix.de
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: t-8ch

scripts/Makefile.extrawarn

@@ -219,6 +219,8 @@ ifneq ($(findstring e, $(KBUILD_EXTRA_WARN))$(CONFIG_WERROR),)
 KBUILD_CPPFLAGS		+= -Werror
 KBUILD_AFLAGS		+= -Wa,--fatal-warnings
 KBUILD_LDFLAGS		+= --fatal-warnings
+KBUILD_USERCFLAGS	+= -Werror
+KBUILD_USERLDFLAGS	+= -Wl,--fatal-warnings
 KBUILD_RUSTFLAGS	+= -Dwarnings
 
 endif