net: sockets: Make NET_SOCKETS_POSIX_NAMES depend on !POSIX_API

These options are mutually exclusive, or more specifically,
CONFIG_POSIX_API has wider scope and supersedes
CONFIG_NET_SOCKETS_POSIX_NAMES. Implementation-wise, the two
options should not be defined at the same time, as that may
lead to declaration conflicts.

Fixes: #16141

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