net: sockets: Use struct timeval provided by libc

Instead of redefining own `struct zsock_timeval` type at the socket
layer, use a standard type provided by libc. This prevents the
compliation errors when application includes both, `net/socket.h` and
standard C header defining `struct timeval` (sys/time.h).

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>