tests: Fix building of unittest/stattest.cc.

This file wasn't including eventq.hh which it happened to be getting
transitively before.

Change-Id: I8ce5572651ddd59160d84794bf5efc90c82d83e6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39564
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/unittest/stattest.cc

@@ -37,6 +37,7 @@
 #include "base/statistics.hh"
 #include "base/types.hh"
 #include "sim/core.hh"
+#include "sim/eventq.hh"
 #include "sim/init.hh"
 #include "sim/stat_control.hh"