fix "headercheck":

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

code/MaterialSystem.h

@@ -45,6 +45,10 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #ifndef AI_MATERIALSYSTEM_H_INC
 #define AI_MATERIALSYSTEM_H_INC
 
+#include <stdint.h>
+
+struct aiMaterial;
+
 namespace Assimp	{
 
 // ------------------------------------------------------------------------------