net: samples: Fix RFC3849 address config option.

NET_TESTING_USE_RFC3849_ADDRESSES not defined as a Kconfig option.
Enable this in Makefile like this
ccflags-y +=-DNET_TESTING_USE_RFC3849_ADDRESSES=1 incase if you need
it.

Change-Id: I96b05d069cf5baa56cc27489b3c38b84412eec97
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>