Explicit InsetTableCell copy constructor

Use explicit default syntax (C++11).

Adapted from:
commit bff97ba76d
commit 3d46cc302b

src/insets/InsetTabular.h

@@ -53,6 +53,9 @@ class InsetTableCell : public InsetText
 public:
 	///
 	InsetTableCell(Buffer * buf);
+	/// We need this since generation of the default is deprecated
+	/// (since we declare the assignment constuctor below).
+	InsetTableCell(InsetTableCell const & in) = default;
 	///
 	InsetCode lyxCode() const { return CELL_CODE; }
 	///