tests: posix: headers: check some structs only when posix_api=y

Some POSIX header tests assumed that various structures would be
declared even if POSIX_API=n. Actually most meaningful POSIX
structures are guarded by feature test macros defined by
the implementation (i.e. implementation conformance macros).

So only run tests on those structures when CONFIG_POSIX_API=y.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>