[gtkmm] TextConstIter: Add a default constructor.

[Murray Cumming <murrayc src gnome org>]

commit 56090d2295fd5cd62ec27b646a34e16dfaf9e540
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Mar 31 14:17:46 2017 +0200

    TextConstIter: Add a default constructor.
    
    TextIter has this by default, but TextConstIter does not,
    because it has custom constructors.

 gtk/src/textiter.hg |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/gtk/src/textiter.hg b/gtk/src/textiter.hg
index 056bc07..5b6f301 100644
--- a/gtk/src/textiter.hg
+++ b/gtk/src/textiter.hg
@@ -305,6 +305,8 @@ class TextConstIter : public TextIterBase
 {
   _CLASS_GENERIC(TextConstIter, GtkTextIter)
 public:
+  inline TextConstIter() noexcept;
+
   /** A TextIter can be implicitly converted to a TextConstIter.
    */
   inline TextConstIter(const TextIter& other) noexcept;
@@ -401,6 +403,10 @@ TextIter TextIter::operator--(int)
 
 /**** Gtk::TextConstIter ******************************************************/
 
+TextConstIter::TextConstIter() noexcept
+{
+}
+
 TextConstIter::TextConstIter(const TextIter& other) noexcept
 : TextIterBase(other.gobj())
 {