examples/webserver-ipv6: Add UIP_CONF_TCP=1 to DEFINES.

TCP is mandatory for this HTTP server.

Fixes builds for platforms which have TCP turned off by default (mulle)

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
Merge pull request #876 from gebart/for-upstream/webserver-with-tcp

examples/webserver-ipv6: Add UIP_CONF_TCP=1 to DEFINES.