objtool: Fix CONFIG_OBJTOOL_WERROR for vmlinux.o

With (!X86_KERNEL_IBT && !LTO_CLANG && NOINSTR_VALIDATION), objtool runs
on both translation units and vmlinux.o.  With CONFIG_OBJTOOL_WERROR,
the TUs get --Werror but vmlinux.o doesn't.  Fix that.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/4f71ab9b947ffc47b6a87dd3b9aff4bb32b36d0a.1742852846.git.jpoimboe@kernel.org