Fix: C++: incomplete extern C guard transition

Recently, commits to fix SDT issues with extern C
(https://bugs.lttng.org/issues/597) brougt in compile errors when the
tracepoint is defined in the same file where the tracepoint provider is
created.

This was due to the presence of extern C guards in tracepoint-event.h, a
header dedicated to tracepoint probe provider compilation. After commits
"Tracepoint probes don't need extern C", it should have gone away. This
is the main fix done by this patch.

This patch also adds missing extern C guards in ust-error.h and
ust-events.h.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>