Fix enum variants flagged fields writing

[hacker-volodya]

Fixes a error when using flags in enums:

#[derive(TlWrite)]
enum EnumWithFlags {
    Named {
        #[tl(flags)]
        flags: (),
        #[tl(flags_bit = 0)]
        value_1: Option<u32>,
    },
    Unnamed(#[tl(flags)] (), #[tl(flags_bit = 0)] Option<u32>)
}

error[E0614]: type `bool` cannot be dereferenced
   --> test_suite\tests\tl_write.rs:103:14
    |
103 |     #[derive(TlWrite)]
    |              ^^^^^^^
    |
    = note: this error originates in the derive macro `TlWrite` (in Nightly builds, run with -Z macro-backtrace for more info)