Some fixes for build with MinGW

git-svn-id: https://assimp.svn.sourceforge.net/svnroot/assimp/trunk@38 67173fc5-114c-0410-ac8e-9d2fd5bffc1f

code/ObjFileMtlImporter.h

@@ -40,6 +40,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #define OBJFILEMTLIMPORTER_H_INC
 
 #include <vector>
+#include <string>
 
 struct aiColor3D;