posix: Move sys/stat.h to minimal libc

Newlib libc already provides sys/stat.h, so trying to have sys/stat.h
on the level of POSIX subsys inevitable leads to include order and
definition conflicts. Instead (as most of other sys/* includes)
should come from the underlying libc.

While moving, made unrelated change of removing #include <kernel.h>,
to accommodate the change reviewers.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>