std::bitflags declaration does not allow trailing comma

From the docs, this works:

```
bitflags!(
    flags Flags: u32 {
        static FlagA       = 0x00000001,
        static FlagB       = 0x00000010,
        static FlagC       = 0x00000100
    }
)
```

This would not:

```
bitflags!(
    flags Flags: u32 {
        static FlagA       = 0x00000001,
        static FlagB       = 0x00000010,
        static FlagC       = 0x00000100,
    }
)
```
