Orb_slam2: xocde build error

[wuzhentao]

in Xcode9 build ,this error "Allocator::value_type must be same type as value_type"

[seantempesta]

In include/LoopClosing.h, change lines 49 and 50 to:

typedef map<KeyFrame*,g2o::Sim3,std::less<KeyFrame*>,
        Eigen::aligned_allocator<std::pair<KeyFrame* const, g2o::Sim3> > > KeyFrameAndPose;