util,systemc: Update the gem5-within-systemc TLM example code.

util,systemc: Update the gem5-within-systemc TLM example code.

Some class names within gem5 changed in March of last year, and this
code was not updated to match. Change ExternalMaster::Port to
ExternalMaster::ExternalPort, and ExternalSlave::Port to
ExternalSlave::ExternalPort.

Change-Id: I04c0970c4107de3449473c24c7c6f99ada72bbb3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32640
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>