wireguard: selftests: do not install headers on UML

Since 1b620d53 ("kbuild: disable header exports for UML in a
straightforward way"), installing headers fails on UML, so just disable
installing them, since they're not needed anyway on the architecture.

Fixes: b438b3b8

 ("wireguard: selftests: support UML")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>