[dialog-sx-editor] increase EX_CAL_NUM_MONTHS from 6 to 12

because the example calendar can show up to 12 months. This means a
daily SX will show all recurrences in the calendar instead of only 6
months.

gnucash/gnome/dialog-sx-editor.c

@@ -83,7 +83,7 @@ static gint _sx_engine_event_handler_id = -1;
 
 #define NUM_LEDGER_LINES_DEFAULT 6
 
-#define EX_CAL_NUM_MONTHS 6
+#define EX_CAL_NUM_MONTHS 12
 #define EX_CAL_MO_PER_COL 3
 
 #define GNC_D_WIDTH 25