net: net_utils: Move ip_to_string to lib/net_utils.c

net: net_utils: Move ip_to_string to lib/net_utils.c

The function string_to_ip is already in net_utils, which is
compiled unconditionally, but ip_to_string is currently only
accessible if the legacy network stack is selected. This
commit puts ip_to_string in net_utils.c and removes it from the
legacy network code.

Signed-off-by: Adriano Cordova <adrianox@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>