examples/contiki/Makefile: use --depth 1 for cloning Contiki
examples/contiki/Makefile: use --depth 1 for cloning Contiki

This change avoids cloning the entire Contiki history when building
the Contiki server example.

authmillenon/libcoap,authmillenon/libcoap,gebart/libcoap,authmillenon/libcoap,gebart/libcoap,gebart/libcoap