arm64: vdso32: Respect -Werror from kbuild

The compiler flags for the compat vDSO are built manually as they are not
compatible with the ones from kbuild. CONFIG_WERROR is not respected.

Explicitly inherit -Werror from kbuild.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Will Deacon <will@kernel.org>

Author: t-8ch

arch/arm64/kernel/vdso32/Makefile

@@ -61,6 +61,7 @@ VDSO_CFLAGS += -DENABLE_COMPAT_VDSO=1
 # KBUILD_CFLAGS from top-level Makefile
 VDSO_CFLAGS += -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
                -fno-strict-aliasing -fno-common \
+               $(filter -Werror,$(KBUILD_CPPFLAGS)) \
                -Werror-implicit-function-declaration \
                -Wno-format-security \
                -std=gnu11