sim-se: rename Process::setpgid member

sim-se: rename Process::setpgid member

The getter methods to access these types of members do not
have a 'get' string in the method names. To make the interface
a bit more consistent, remove the 'set' part of the member name.

Change-Id: I04c56bd9d9feb1cf68ff50a1152083ea57ea7c62
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20008
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>