depth: Change White's 6th depth from 12 to 15

To fix:

 1.    d6    d2
 2.    f4    f6
 3.    b4    d7
 4.    d1    e4
 5.    a1    g1
 6.    a4  (bad)

Although solution is not draw, but it is more better.

src/mills.cpp

@@ -481,7 +481,7 @@ Depth get_search_depth(const Position *pos)
             constexpr Depth placingDepthTable9[25] = {
                 +1, 1, +1,  1,  /* 0 ~ 3 */
                 +3, 3, +3,  15, /* 4 ~ 7 */
-                +6, 3, +12, 0,  /* 8 ~ 11 */
+                +6, 3, +15, 0,  /* 8 ~ 11 */
                 +0, 0, +0,  0,  /* 12 ~ 15 */
                 +0, 0, +0,  0,  /* 16 ~ 19 */
                 +0, 0, +0,  0,  /* 20 ~ 23 */