[lldb] Require native for command-thread-siginfo.test

command-thread-siginfo.test employs a subject with a call to wait, and
thus requires system-linux. However, it's possible to target non-Linux
platforms despite operating on Linux hosts. So, have it require native
too.

Reviewed By: mgorny, labath

Differential Revision: https://reviews.llvm.org/D121487