mruby-random: fix unary minus on unsigned type warning

replace (-rot) with (32 - rot) to avoid msvc warning c4146. both
expressions are equivalent when masked with & 31, but the latter
is clearer and doesn't trigger warnings about negating unsigned values.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-random/src/random.c

@@ -82,7 +82,7 @@ rand_uint32(rand_state *rng)
   uint32_t rot = (uint32_t)(oldstate >> 59u);
 
   /* Rotate right by rot bits (handles rot=0 case correctly) */
-  return (xorshifted >> rot) | (xorshifted << ((-rot) & 31));
+  return (xorshifted >> rot) | (xorshifted << ((32 - rot) & 31));
 }
 
 #ifndef MRB_NO_FLOAT