Fix silly atomic_flag build warning

Dan Roserequested to merge
rotu/performance_test:patch-6 into master 
In file included from /opt/ros/master/src/ApexAI/performance_test/performance_test/src/communication_abstractions/communicator.cpp:17:
In file included from /opt/ros/master/src/ApexAI/performance_test/performance_test/src/communication_abstractions/communicator.hpp:22:
/opt/ros/master/src/ApexAI/performance_test/performance_test/src/communication_abstractions/../utilities/spin_lock.hpp:29:12: warning: braces around scalar initializer [-Wbraced-scalar-init]
  : m_lock(ATOMIC_FLAG_INIT) {}
           ^~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/atomic_base.h:157:26: note: expanded from macro 'ATOMIC_FLAG_INIT'
#define ATOMIC_FLAG_INIT { 0 }
                         ^~~~~
1 warning generated.
In file included from /opt/ros/master/src/ApexAI/performance_test/performance_test/src/data_running/data_runner_factory.cpp:28:
In file included from /opt/ros/master/src/ApexAI/performance_test/performance_test/src/data_running/../communication_abstractions/fast_rtps_communicator.hpp:33:
In file included from /opt/ros/master/src/ApexAI/performance_test/performance_test/src/data_running/../communication_abstractions/communicator.hpp:22:
/opt/ros/master/src/ApexAI/performance_test/performance_test/src/data_running/../communication_abstractions/../utilities/spin_lock.hpp:29:12: warning: braces around scalar initializer [-Wbraced-scalar-init]
  : m_lock(ATOMIC_FLAG_INIT) {}
           ^~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/atomic_base.h:157:26: note: expanded from macro 'ATOMIC_FLAG_INIT'
#define ATOMIC_FLAG_INIT { 0 }
                         ^~~~~
1 warning generated.

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