Include kNet memory leak check only on MSVC to fix possible build error.

Source/ThirdParty/kNet/include/kNet/DebugMemoryLeakCheck.h

@@ -16,7 +16,10 @@
 /** @file DebugMemoryLeakCheck.h
 	@brief Provides overloads of operators new and delete for tracking memory leaks. */
 
-#if defined (WIN32) && defined(_DEBUG) && defined(KNET_MEMORY_LEAK_CHECK)
+// Modified by Lasse Oorni for Urho3D
+
+// Urho3D: only include on MSVC
+#if defined(_MSC_VER) && defined(_DEBUG) && defined(KNET_MEMORY_LEAK_CHECK)
 
 #include <new>
 #include <crtdbg.h>