PERF: fix cppcheck perf warning

[Modules/Core/Common/include/itkAnnulusOperator.h:98]: (performance) Variable 'm_Spacing' is assigned in constructor body. Consider performing initialization in initialization list.

Change-Id: I4d3544169f8483da962d8fe280b2f6b61f9f263d
Merge topic 'cppcheck-annulus'

6a6a646 PERF: fix cppcheck perf warning