rand: fix leaks with -autofree

vlib/rand/pcg32/pcg32.v

@@ -224,3 +224,9 @@ pub fn (mut rng PCG32RNG) f64_in_range(min f64, max f64) f64 {
 	}
 	return min + rng.f64n(max - min)
 }
+
+// free should be called when the generator is no longer needed
+[unsafe]
+pub fn (mut rng PCG32RNG) free() {
+	unsafe { free(rng) }
+}