missing include

Former-commit-id: a99bedb77ae74eec080a4e6fc1e95423db898759

include/igl/copyleft/cgal/point_mesh_squared_distance.cpp

@@ -7,6 +7,7 @@
 // obtain one at http://mozilla.org/MPL/2.0/.
 #include "point_mesh_squared_distance.h"
 #include "mesh_to_cgal_triangle_list.h"
+#include "assign_scalar.h"
 
 template <
   typename Kernel,