[gtksourceviewmm] Add m4 ustring conversions to fix the build.

[Murray Cumming <murrayc src gnome org>]

commit 6cda087003de21b76d73a59a4a81b3cd3859a242
Author: Murray Cumming <murrayc murrayc com>
Date:   Sun Jan 14 21:06:25 2018 +0100

    Add m4 ustring conversions to fix the build.
    
    Needed by recent gmmproc versions, because there is no good default.

 gtksourceview/src/completionproposal.hg |    1 +
 gtksourceview/src/completionprovider.hg |    1 +
 2 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/gtksourceview/src/completionproposal.hg b/gtksourceview/src/completionproposal.hg
index 0ff8879..9068c5f 100644
--- a/gtksourceview/src/completionproposal.hg
+++ b/gtksourceview/src/completionproposal.hg
@@ -144,6 +144,7 @@ public:
 
 private:
 
+#m4 _CONVERSION(`char*',`Glib::ustring',`Glib::convert_return_gchar_ptr_to_ustring($3)')
   _WRAP_VFUNC(Glib::ustring get_label() const, get_label)
 
   _WRAP_VFUNC(Glib::ustring get_markup() const, get_markup)
diff --git a/gtksourceview/src/completionprovider.hg b/gtksourceview/src/completionprovider.hg
index a9a8e83..b20d792 100644
--- a/gtksourceview/src/completionprovider.hg
+++ b/gtksourceview/src/completionprovider.hg
@@ -198,6 +198,7 @@ public:
 
 private:
 
+#m4 _CONVERSION(`char*',`Glib::ustring',`Glib::convert_return_gchar_ptr_to_ustring($3)')
   _WRAP_VFUNC(Glib::ustring get_name() const, get_name)
 
   _WRAP_VFUNC(Glib::RefPtr<Gdk::Pixbuf> get_icon(), get_icon, refreturn)