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>