Make the legacy inline headers compile standalone.

It's useful to have the legacy inlines compile by themselves, both to
make header unification easier, and to ensure that the inline versions
match the regular declarations. Notably, this wasn't true for
sigismember, which took a const sigset_t* in the regular header, and
sigset_t* in the inline version.

Bug: http://b/28178111
Change-Id: Id8a3b7dcb1bfa61eed93c9fb50d3192744f8bef5