added .step extension to IFC loader

The extension .step is at least as common as .stp, so both should be supported.

code/AssetLib/IFC/IFCLoader.cpp

@@ -115,7 +115,7 @@ static const aiImporterDesc desc = {
     0,
     0,
     0,
-    "ifc ifczip stp"
+    "ifc ifczip step stp"
 };
 
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