Update M3DWrapper.h

Add missing include.
Odd that MSVC2013-2017, GCC and Clang were ok with this.

code/M3D/M3DWrapper.h

@@ -50,6 +50,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include <memory>
 #include <vector>
+#include <string>
 
 #include "m3d.h"