systemc: Define the nonstandard sc_event_queue_port typedef.

Used in the tests.

Change-Id: Ic4b4a313b2bd02e5cb3ec07c4f7ee79219728881
Reviewed-on: https://gem5-review.googlesource.com/11262
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>