examples/ipv6/rpl-border-router: Add -DUIP_CONF_TCP=1 to CFLAGS

The example used to rely on the default value being set to 1 which
caused build failures on platforms which have a default UIP_CONF_TCP=0

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
Merge pull request #877 from gebart/for-upstream/rpl-border-router-webserver-tcp

examples/ipv6/rpl-border-router: Add -DUIP_CONF_TCP=1 to CFLAGS