MeshAlgo : Add missing includes

This fixes compilation on MacOS.

Author: johnhaddon

src/IECoreScene/MeshAlgoSegment.cpp

@@ -39,6 +39,8 @@
 #include "IECoreScene/MeshAlgo.h"
 #include "IECoreScene/private/PrimitiveAlgoUtils.h"
 
+#include <unordered_map>
+
 using namespace Imath;
 using namespace IECore;
 using namespace IECoreScene;

src/IECoreScene/MeshAlgoSplit.cpp

@@ -37,6 +37,8 @@
 
 #include "IECoreScene/MeshAlgo.h"
 
+#include <unordered_map>
+
 using namespace Imath;
 using namespace IECore;
 using namespace IECoreScene;