objtool: Fix obsolete reference to CONFIG_X86_SMAP

CONFIG_X86_SMAP no longer exists.  For objtool's purposes it has been
replaced with CONFIG_HAVE_UACCESS_VALIDATION.

Fixes: 03f16cd0 ("objtool: Add CONFIG_OBJTOOL")
Reported-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/44c57668768c1ba1b4ba1ff541ec54781636e07c.1654101721.git.jpoimboe@kernel.org