x86/extable: Annotate ex_handler_msr_mce() as a dead end

Fix

  vmlinux.o: warning: objtool: fixup_exception+0x2d6: unreachable instruction

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220520192729.23969-1-bp@alien8.de