added missing includes in gradmat

Former-commit-id: efe524bef565d2486b071cb3e51b1e831b011392

include/igl/gradMat.cpp

@@ -1,4 +1,5 @@
 #include "gradMat.h"
+#include <vector>
 
 template <typename T, typename S>
 IGL_INLINE void igl::gradMat(const Eigen::Matrix<T, Eigen::Dynamic, Eigen::Dynamic> &V,

include/igl/gradMat.h

@@ -1,15 +1,9 @@
-//
-//  gradMat.h
-//
-//  Created by Christian Schüller on 07/03/13.
-//  Copyright (c) 2013 ETHZ IGL. All rights reserved.
-//
-
 #ifndef IGL_GRAD_MAT_H
 #define IGL_GRAD_MAT_H
 #include "igl_inline.h"
 
 #include <Eigen/Core>
+#include <Eigen/Sparse>
 
 namespace igl {
   // GRAD