Move .git-commit-id.h include to where it's actually referenced in
scp.c to reduce the recompile overhead for build systems that track
build dependencies.

Ensure that headers that reference SIM_MAJOR include sim_rev.h as a
defensive coding practice.

Net effect: Reduces recompiled source to a minimum when .git-commit-id.h
is updated and removes the sim_defs.h > sim_rev.h > .git-commit-id.h
dependency subgraph. Everything includes sim_defs.h, so a change to
.git-commit-id.h causes a large recompile, vice what should just be a
scp.c recompile and simulator suite relink.

Does not impact makefile builds, since makefile builds will recompile
everything; dependency tracking is superfluous.