base-stats: Rename Units namespace as units

As part of recent decisions regarding namespace
naming conventions, all namespaces will be changed
to snake case.

Stats::Units became Stats::units.

Change-Id: I9ce855b291db122d952098a090a2984b42152850
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45415
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>

src/sim/process.cc

@@ -127,7 +127,7 @@ Process::Process(const ProcessParams &params, EmulationPageTable *pTable,
       fds(std::make_shared<FDArray>(
                   params.input, params.output, params.errout)),
       childClearTID(0),
-      ADD_STAT(numSyscalls, Stats::Units::Count::get(),
+      ADD_STAT(numSyscalls, Stats::units::Count::get(),
                "Number of system calls")
 {
     if (_pid >= System::maxPID)