libc: minimal: Add headers as system includes

This is consistent with how newlib headers are treated, and will
have effect of ninlibc headers to be further down in the include
order. This is important, because some POSIX subsys headers
override those of libc. Without this change, we can't streamline
POSIX build config using zephyr_interface_library_named() cmake
directive, because includes will be in wrong order.

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