COMP: export required in explicitly defined NumbericTraits consts

When shared libraries are enabled with clang 5.0 on OSX, the
explicitly defined NumericTraits constants such as:
itk::NumericTraits<itk::Vector<double, 3u> >::Zero
were not being exported in shared libraries, causing link errors.

The export specification is required at explicit definition and
declaration.

Change-Id: Ic850a721ff7f9e41e99c7301a3415bce20a42068
Merge branch 'FixClang5NumericTraitConstantsExport' into release

Change-Id: If8e3fcf8e4434899d512194393b759a84a3ae339