[xkbcommon] Make C++ happy.

[Bill Spitzak]

Wander Lairson Costa wrote:

>  /** Flags for keymap compilation. */
>  enum xkb_keymap_compile_flags {
> +    /** Do not apply any flag. */
> +    XKB_MAP_COMPILE_NO_FLAG = 0,
>      /** Apparently you can't have empty enums.  What a drag. */
>      XKB_MAP_COMPILE_PLACEHOLDER = 0
>  };

I think you can remove the PLACEHOLDER