fixed include issue in viewer

Former-commit-id: 9dcbbc6c484028d3f9b54c0ce5f74a894b9cc948

include/igl/viewer/Viewer.cpp

@@ -32,7 +32,9 @@
 #include <Eigen/LU>
 
 #define GLFW_INCLUDE_GLU
-//#define GLFW_INCLUDE_GLCOREARB
+#ifndef _WIN32
+  #define GLFW_INCLUDE_GLCOREARB
+#endif
 #include <GLFW/glfw3.h>
 
 #include <cmath>