missing eigen include

Former-commit-id: be6c9a64551c7cb980a387db6737f3d0a0980288

include/igl/dfs.h

@@ -1,6 +1,7 @@
 #ifndef IGL_DFS_H
 #define IGL_DFS_H
 #include "igl_inline.h"
+#include <Eigen/Core>
 #include <vector>
 namespace igl
 {