STYLE: Fix "sign-compare" warning in vtkMRMLColorLogic

This commit fixes a warning introduced by r24742 (ENH: Add terminology
information to color picker in Editor)

/path/to/Slicer/Libs/MRML/Logic/vtkMRMLColorLogic.cxx:101:45: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
for (int i = 0; i < this->ColorFiles.size(); i++)
^
/path/to/Projects/Slicer/Libs/MRML/Logic/vtkMRMLColorLogic.cxx:106:49: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
for (int i = 0; i < this->UserColorFiles.size(); i++)
^
/path/to/Projects/Slicer/Libs/MRML/Logic/vtkMRMLColorLogic.cxx:112:56: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
for (int i = 0; i < this->TerminologyColorFiles.size(); i++)
^

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24744 3bd1e089-480b-0410-8dfb-8563597acbee