Make lttng-ust robust against -finstrument-functions.

[ Edit by Mathieu Desnoyers:

We need to declare the no_instrument_function attribute on function
declarations (rather than definition) for g++. Moved the attribute prior
to the function declaration (rather than after) to follow the coding
style within LTTng-UST. ]

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