net/dhcpv4: Re-instate init_app()

Re-instate the init_app()call which was accidentally removed by commit
commit 34f9391c9cf5 ("dhcpv4: Report address acquisition.")

Change-Id: Ic5ef70fd855a5bcb3ea1171de652994db493f4bb
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>

Author: mshawcroft

samples/net/dhcpv4_client/src/main.c

@@ -79,5 +79,7 @@ void main(void)
 {
 	NET_INFO("In main");
 
+	init_app();
+
 	main_fiber();
 }