kbuild: disable header exports for UML in a straightforward way

Previously 'make ARCH=um headers' stopped because of missing
arch/um/include/uapi/asm/Kbuild.

The error is not shown since commit ed102bf2 ("um: Fix W=1
missing-include-dirs warnings") added arch/um/include/uapi/asm/Kbuild.

Hard-code the unsupported architecture, so it works like before.

Fixes: ed102bf2

 ("um: Fix W=1 missing-include-dirs warnings")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Richard Weinberger <richard@nod.at>