scons: Use HAVE_PROTOC when building protobuf files.

Also wrap PROTOC in {}s which is better form.

Change-Id: I3f80c260593a1d5b7fb5394fe4b71cb774e652ce
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22884
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed