correct header guard

Former-commit-id: 64e793ef811200df20bdbdc8d5f68f1062a6a1a6

include/igl/vector_area_matrix.h

@@ -5,8 +5,8 @@
 // This Source Code Form is subject to the terms of the Mozilla Public License
 // v. 2.0. If a copy of the MPL was not distributed with this file, You can
 // obtain one at http://mozilla.org/MPL/2.0/.
-#ifndef IGL_AREAMATRIX_H
-#define IGL_AREAMATRIX_H
+#ifndef IGL_VECTOR_AREA_MATRIX_H
+#define IGL_VECTOR_AREA_MATRIX_H
 #include "igl_inline.h"
 
 #include <Eigen/Dense>