month-view: initialize variable

Some compilers seem to be over-pessimistic and abort the build with:

 gcal-month-view.c: In function 'gcal_month_view_drag_drop':
 gcal-month-view.c:908:32: error: 'timespan' may be used uninitialized in this function [-Werror=maybe-uninitialized]
            GDateTime *new_end = g_date_time_add (new_start, timespan);

Even though the usage of the variable is guarded by an equal if as the assignment

https://bugzilla.gnome.org/show_bug.cgi?id=770667

src/gcal-month-view.c

@@ -865,7 +865,7 @@ gcal_month_view_drag_drop (GtkWidget      *widget,
   gint cell, diff;
   gint start_month, current_month;
   gint start_year, current_year;
-  GTimeSpan timespan;
+  GTimeSpan timespan = 0;
 
   cell = get_dnd_cell (widget, x, y);
   event_widget = gtk_drag_get_source_widget (context);