gdb: update the copyright year in async-event.[ch]

gdb: update the copyright year in async-event.[ch]

The async-event.[ch] files were introduced recently as a result of
splitting the event-loop.  I believe the copyright year update was
just an oversight.  So, this patch fixes that.

gdb/ChangeLog:
2020-05-13  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* async-event.c: Update the copyright year.
	* async-event.h: Update the copyright year.