network: use link_start_dhcp4_server() at one more place

Otherwise, even if the persistent storage is not ready, the DHCP server
may be started e.g. by unplugging and plugging cable.

Follow-up for 5582b36c38.

src/network/networkd-dhcp-server.c

@@ -144,7 +144,7 @@ int network_adjust_dhcp_server(Network *network, Set **addresses) {
         return 0;
 }
 
-static int link_start_dhcp4_server(Link *link) {
+int link_start_dhcp4_server(Link *link) {
         int r;
 
         assert(link);