bgpd: fix show run of network route-distinguisher

bgpd: fix show run of network route-distinguisher

Route-distinguisher (RD) is not printed properly in show run:

>  address-family ipv6 vpn
>   network ff01::/64 rd (null) label 7
>   network ff01::/64 rd (null) label 8

ad151f66aa ("bgpd: Refactor bgp_static_set/bgp_static_set_safi") merged
bgp_static_set_safi into bgp_static_set but inadvertently omitted the
handling of prd_pretty.

Copy the pretty RD string if available.

> address-family ipv6 vpn
>  network ff01::/64 rd 75:5 label 7
>  network ff01::/64 rd 85:5 label 8

Fixes: ad151f66aa ("bgpd: Refactor bgp_static_set/bgp_static_set_safi")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>