[gtksourceviewmm] Add m4 _CONVERSION()s to fix build

[Murray Cumming <murrayc src gnome org>]

commit ceb92b50698300dd988204d68e405f66c1383f6a
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Sep 27 22:51:14 2019 +0200

    Add m4 _CONVERSION()s to fix build
    
    These are needed by recent glibmm versions.

 gtksourceview/src/completionproposal.hg | 1 +
 gtksourceview/src/completionprovider.hg | 1 +
 2 files changed, 2 insertions(+)
---
diff --git a/gtksourceview/src/completionproposal.hg b/gtksourceview/src/completionproposal.hg
index 9068c5f..cabeb81 100644
--- a/gtksourceview/src/completionproposal.hg
+++ b/gtksourceview/src/completionproposal.hg
@@ -50,6 +50,7 @@ public:
    *
    * @newin{2,10}
    */
+#m4 _CONVERSION(`Glib::ustring', `char*', `g_strdup(($3).c_str())')
   _WRAP_METHOD(Glib::ustring get_label() const, gtk_source_completion_proposal_get_label)
 
   /** Gets the label of a proposal with markup. The label is shown in the list
diff --git a/gtksourceview/src/completionprovider.hg b/gtksourceview/src/completionprovider.hg
index b20d792..f95ee14 100644
--- a/gtksourceview/src/completionprovider.hg
+++ b/gtksourceview/src/completionprovider.hg
@@ -199,6 +199,7 @@ public:
 private:
 
 #m4 _CONVERSION(`char*',`Glib::ustring',`Glib::convert_return_gchar_ptr_to_ustring($3)')
+#m4 _CONVERSION(`Glib::ustring', `char*', `g_strdup(($3).c_str())')
   _WRAP_VFUNC(Glib::ustring get_name() const, get_name)
 
   _WRAP_VFUNC(Glib::RefPtr<Gdk::Pixbuf> get_icon(), get_icon, refreturn)