Move XmlBase destructor implementation to source file

This creates an out-of-line virtual method definition. This removes an instance of the Clang warning `-Wweak-vtables`.

NAS2D/Xml/XmlBase.cpp

@@ -16,6 +16,11 @@ using namespace NAS2D::Xml;
 bool XmlBase::condenseWhiteSpace = true;
 
 
+XmlBase::~XmlBase()
+{
+}
+
+
 /**
  * Get the row of the node in the document.
  *

NAS2D/Xml/XmlBase.h

@@ -56,7 +56,7 @@ class XmlBase
 	XmlBase() = default;
 	XmlBase(const XmlBase&) = delete;
 	void operator=(const XmlBase& base) = delete;
-	virtual ~XmlBase() = default;
+	virtual ~XmlBase();
 
 	/**
 	 * Writes the XML entity to a string buffer.