net: zperf: Fix SO_RCVTIMEO dependency

Since 6c30c9ac (samples: net: zperf:
Rewrite upload part to use sockets), zperf uses SO_RCVTIMEO in UDP mode,
hence must depend on/enable support for it.

Without SO_RCVTIMEO support, zperf fails like this:
> nb_packets=47   delay=188964    adjust=-13
> setsockopt error (109)
> setsockopt error (109)
> -
> Upload completed!
> LAST PACKET NOT RECEIVED!!!
> Statistics:             server  (client)
> Duration:               0 us    (10.19 s)
> Num packets:            0       (50)
> Num packets out order:  0
> Num packets lost:       0
> Jitter:                 0 us
> Rate:                   0 Kbps  (9 Kbps)

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>