Fixed missing imports on rotation cipher exercise (#449)

[no important files changed]

Author: sinon

exercises/practice/rotational-cipher/rotational_cipher.zig

@@ -1,3 +1,6 @@
+const std = @import("std");
+const mem = std.mem;
+
 pub fn rotate(allocator: mem.Allocator, text: []const u8, shiftKey: u5) mem.Allocator.Error![]u8 {
     _ = allocator;
     _ = text;