samples: net: echo-server: Fix conditional around net_if_ipv4_addr_add()

The invocation of net_if_ipv4_addr_add() should be included within the
conditional code that sets up the address added.

Change-Id: I9b4e76a6b4922b3455bc6b383431b4124d40a49e
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>