gdb: small cleanup of async-event.c structs

gdb: small cleanup of async-event.c structs

This is a small cleanup to normalize the structures in async-event.c
with the rest of the code base:

- Remove the unnecessary typedefs
- Fix indentation of struct bodies
- Put comments above fields

No functional changes expected.

gdb/ChangeLog:

	* async-event.c (struct async_signal_handler, struct
	async_event_handler): Reformat, remove typedef.