Account for double pawn push in evaluate_unstoppable_pawns()

One of the most nitpicking patches I have ever seen.

Of course almost no functional change, but added just
becasue we are very pedantic ;-)

Spotted by Marek Kwiatkowski

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/evaluate.cpp

@@ -914,6 +914,7 @@ namespace {
             Square s = pop_1st_bit(&b);
             Square queeningSquare = relative_square(c, make_square(square_file(s), RANK_8));
             int d =  square_distance(s, queeningSquare)
+                   - (relative_rank(c, s) == RANK_2) // Double pawn push
                    - square_distance(pos.king_square(opposite_color(c)), queeningSquare)
                    + int(c != pos.side_to_move());