Replace virtual with override where possible.

Using clang-tidy

test/ModulesTest.cpp

@@ -20,8 +20,8 @@
 
 class ModulesTest : public ::testing::Test {
   protected:
-    void SetUp() { CZNC::CreateInstance(); }
-    void TearDown() { CZNC::DestroyInstance(); }
+    void SetUp() override { CZNC::CreateInstance(); }
+    void TearDown() override { CZNC::DestroyInstance(); }
 };
 
 class CLegacyModule : public CModule {