kmswebrtcendpoint: Fix symbol names to not use the libnice namespace

libnice has reserved the ‘nice_’ namespace for itself, so Kurento should
not prefix its symbols with that. This was causing linking failures when
compiling against libnice ≥ 0.1.5, which has introduced a
nice_agent_recv() function upstream.

Change-Id: Iff0b1c02cbc0c9378f6197d5bece2ba26831d39d