Move MapEditEventAreaIgnorer to emerge.cpp

It's only used in emerge threads and it's a local object, don't expose it to the whole Minetest

src/emerge.cpp

@@ -85,6 +85,31 @@ private:
 	friend class EmergeManager;
 };
 
+class MapEditEventAreaIgnorer
+{
+public:
+	MapEditEventAreaIgnorer(VoxelArea *ignorevariable, const VoxelArea &a):
+		m_ignorevariable(ignorevariable)
+	{
+		if(m_ignorevariable->getVolume() == 0)
+			*m_ignorevariable = a;
+		else
+			m_ignorevariable = NULL;
+	}
+
+	~MapEditEventAreaIgnorer()
+	{
+		if(m_ignorevariable)
+		{
+			assert(m_ignorevariable->getVolume() != 0);
+			*m_ignorevariable = VoxelArea();
+		}
+	}
+
+private:
+	VoxelArea *m_ignorevariable;
+};
+
 ////
 //// EmergeManager
 ////