fix a compilation pb with clang (it's actually surprising gcc did not complain)
fix a compilation pb with clang (it's actually surprising gcc did not complain)

pthulhu/eigen,cjntaylor/eigen,cjntaylor/eigen,pthulhu/eigen,cjntaylor/eigen,pthulhu/eigen,pthulhu/eigen,pthulhu/eigen,cjntaylor/eigen