samples: wpanusb: remove unnecessary condition for ccflags

The condition in src/Makefile is not necessary since the
net_private.h is always included from wpanusb.c and is not
dependent on the transceiver.

Change-Id: I07a0bf599f328bacb8c5677f20eb221a05f30866
Signed-off-by: Johann Fischer <j.fischer@phytec.de>