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 :

 In file included from session.hpp:18:0,
                  from lttng-sessiond.hpp:21,
                  from utils.cpp:10:
 ../../../src/common/pthread-lock.hpp: In constructor 'lttng::pthread::details::mutex::mutex(pthread_mutex_t&)':
 ../../../src/common/pthread-lock.hpp:29:61: error: invalid initialization of non-const reference of type 'pthread_mutex_t&' from an rvalue of type '<brace-enclosed initializer list>'
   explicit mutex(pthread_mutex_t& mutex_p) : _mutex{ mutex_p }

Use oldstyle initialization of references instead.

Change-Id: I2fd756b48fd8e70e30199c86527c025117bfb6ff
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>