std: change @import("builtin") to std.builtin

lib/std/unicode.zig

@@ -4,7 +4,7 @@
 // The MIT license requires this copyright notice to be included in all copies
 // and substantial portions of the software.
 const std = @import("./std.zig");
-const builtin = @import("builtin");
+const builtin = std.builtin;
 const assert = std.debug.assert;
 const testing = std.testing;
 const mem = std.mem;