Minor update to avoid duplicated #define statement.

Former-commit-id: de9257897c7b5294abbb890c91fabf30ecc67d1e

include/igl/slice.h

@@ -9,7 +9,9 @@
 #define IGL_SLICE_H
 #include "igl_inline.h"
 
+#ifndef EIGEN_YES_I_KNOW_SPARSE_MODULE_IS_NOT_STABLE_YET
 #define EIGEN_YES_I_KNOW_SPARSE_MODULE_IS_NOT_STABLE_YET
+#endif
 #include <Eigen/Sparse>
 
 namespace igl