samples: http_get: move 'not CONFIG_NATIVE_LIBC' filter to POSIX build

'not CONFIG_NATIVE_LIBC' filter is just to prevent building for
'native_posix' platform with additional options selected by
'sample.net.sockets.http_get.posix' testcase.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>