Build fix: brace-enclosed initlializer lists error with g++ 4.8

Build fix: brace-enclosed initlializer lists error with g++ 4.8

A build error occurs when building using g++ 4.8 :

rotation-thread.cpp: In constructor 'lttng::sessiond::rotation_thread::rotation_thread(lttng::sessiond::rotation_thread_timer_queue&, notification_thread_handle&)':
rotation-thread.cpp:400:58: error: invalid initialization of non-const reference of type 'lttng::sessiond::rotation_thread_timer_queue&' from an rvalue of type '<brace-enclosed initializer list>'
  _notification_thread_handle{ notification_thread_handle }

Use old-style initialization of references instead.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ia3392a88b8a2d8dd8c60330c16229f507338e7cd