Fix bug in rebuild occ

We need to init the variables in the loops first.
int p=p doesn't work.

engine/src/bitboard.c

@@ -604,8 +604,8 @@ void rebuild_occ(struct Board *board) {
     uint64_t white=0;
     uint64_t black=0;
 
-    for (int p = P; p <= K; ++p) white |= board->pieces[p];
-    for (int p = p; p <= k; ++p) black |= board->pieces[p];
+    for (int i = P; i <= K; ++i) white |= board->pieces[i];
+    for (int i = p; i <= k; ++i) black |= board->pieces[i];
 
     board->occ[WHITE] = white;
     board->occ[BLACK] = black;