Fix compilation error with eigen 3.4.0
Fix compilation error with eigen 3.4.0

Although I have been said that this issue is going to be eventually fixed in eigen 3.4.1, if I'm not mistaken the size of that AᵀA matrix is going to be always 9×9, so it's better being explicit.
openMVG/openMVG,openMVG/openMVG,openMVG/openMVG,openMVG/openMVG,openMVG/openMVG