Add an 'override' specifier to testrunner

testrunner.cc's {anonymous}::Environment::SetUp() needs an override
specifier since testing::Environment::SetUp() is virtual.

Bug: None
Test: CXX=clang++ make tests
Change-Id: I45dc8851971d8fc067d9ecb80d98deae85a6f30c
diff --git a/testrunner.cc b/testrunner.cc
index b848c9c..62ef6d7 100644
--- a/testrunner.cc
+++ b/testrunner.cc
@@ -28,7 +28,7 @@
  public:
   ~Environment() override = default;
 
-  void SetUp() {
+  void SetUp() override {
     init_logging(LOG_TO_FD, STDERR_FILENO, LOG_INFO);
   }
 };