Fix strict ISO-C compatibility for ust-tracepoint-event.h public header

Fix strict ISO-C compatibility for ust-tracepoint-event.h public header

../../include/lttng/ust-tracepoint-event.h:328:23: warning: ISO C does not permit named variadic macros [-Wvariadic-macros]

Use ... and __VA_ARGS__ instead of args... and args.

This enables ISO-C compability for the tracepoint headers for program
instrumentation. Note that the probes need to be built _without_ strict
C99 flags (they require gnu extensions).

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