st/password-entry: Initialize :show-peek-icon property

The icon is shown by default, but the struct member that backs the
property starts out as FALSE.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/910

src/st/st-password-entry.c

@@ -164,6 +164,8 @@ st_password_entry_init (StPasswordEntry *entry)
                                            NULL);
   st_entry_set_secondary_icon (ST_ENTRY (entry), priv->peek_password_icon);
 
+  priv->show_peek_icon = TRUE;
+
   clutter_text = st_entry_get_clutter_text (ST_ENTRY (entry));
   clutter_text_set_password_char (CLUTTER_TEXT (clutter_text), BLACK_CIRCLE);