Minor C++-ification in ravenscar-thread.c

Minor C++-ification in ravenscar-thread.c

This changes some functions in ravenscar-thread.c to return "bool"
rather than int, where appropriate, and also changes "(void)" to "()".

gdb/ChangeLog
2019-02-15  Tom Tromey  <tromey@adacore.com>

	* ravenscar-thread.c (is_ravenscar_task)
	(ravenscar_task_is_currently_active): Return bool.
	(ravenscar_update_inferior_ptid, get_running_thread_msymbol)
	(_initialize_ravenscar): Remove "(void)".
	(has_ravenscar_runtime, ravenscar_runtime_initialized): Likewise.
	Return bool.