Fix: multiple providers in same C file failure

If we have a C file in an application that is meant to contain both the
instrumentation and the provider, build fails if we have more than one
provider.

Fixed by putting back definitions to the state they were when defined by
tracepoint.h after a probe provider is created. This allows the next
tracepoint provider to define its tracepoints.

Fixes #805

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