fix "headercheck":

part of https://github.com/assimp/assimp/issues/534

code/OptimizeGraph.h

@@ -47,6 +47,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include "BaseProcess.h"
 #include "ProcessHelper.h"
 #include "../include/assimp/types.h"
+#include <set>
 
 struct aiMesh;
 class OptimizeGraphProcessTest;