added const to kdtree node to fix compile error in <algorithm> on XCode/c++11/10.8 sdk
added const to kdtree node to fix compile error in <algorithm> on XCode/c++11/10.8 sdk

SoylentGraham/libmv,SoylentGraham/libmv,SoylentGraham/libmv,SoylentGraham/libmv