Pyrrhic: Fix mixed-signedness comparison (#678)

GCC 13 has made checks for mixed-signedness comparisons more stringent,
causing a build failure on Ubuntu 23.10 (pre-release). Workaround this
by adding some casts in the Pyrrhic code.

Fixes #677

No functional change.

src/pyrrhic/tbchess.c

@@ -106,7 +106,7 @@ static uint64_t pyrrhic_pieces_by_type(const PyrrhicPosition *pos, int colour, i
 
 static int pyrrhic_char_to_piece_type(char c) {
 
-    for (int i = PYRRHIC_PAWN; i <= PYRRHIC_KING; i++)
+    for (int i = PYRRHIC_PAWN; i <= (int)PYRRHIC_KING; i++)
         if (c == pyrrhic_piece_to_char[i])
             return i;
     return 0;

src/pyrrhic/tbprobe.c

@@ -445,13 +445,13 @@ static void prt_str(const PyrrhicPosition *pos, char *str, int flip) {
 
     int color = flip ? PYRRHIC_BLACK : PYRRHIC_WHITE;
 
-    for (int pt = PYRRHIC_KING; pt >= PYRRHIC_PAWN; pt--)
+    for (int pt = PYRRHIC_KING; pt >= (int)PYRRHIC_PAWN; pt--)
         for (int i = PYRRHIC_POPCOUNT(pyrrhic_pieces_by_type(pos, color, pt)); i > 0; i--)
             *str++ = pyrrhic_piece_to_char[pt];
 
     *str++ = 'v';
 
-    for (int pt = PYRRHIC_KING; pt >= PYRRHIC_PAWN; pt--)
+    for (int pt = PYRRHIC_KING; pt >= (int)PYRRHIC_PAWN; pt--)
         for (int i = PYRRHIC_POPCOUNT(pyrrhic_pieces_by_type(pos, color^1, pt)); i > 0; i--)
             *str++ = pyrrhic_piece_to_char[pt];
     *str++ = 0;