COMP: Fix sign-compare warning in qSlicerApplication

This commit fixes the following warning:

/path/to/Slicer/Base/QTGUI/qSlicerApplication.cxx: In member function ‘void qSlicerApplication::setupFileLogging()’:
/path/to/Slicer/Base/QTGUI/qSlicerApplication.cxx:626:46: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
for (unsigned int fileNumber=0; fileNumber<numberOfFilesToKeep; ++fileNumber)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23602 3bd1e089-480b-0410-8dfb-8563597acbee