input_common: Treat warnings as errors

Migrates over warnings as errors for input common to match how the
common library treats warnings as errors.

src/input_common/gcadapter/gc_poller.cpp

@@ -96,7 +96,6 @@ std::unique_ptr<Input::ButtonDevice> GCButtonFactory::Create(const Common::Param
                                               adapter.get());
     }
 
-    UNREACHABLE();
     return nullptr;
 }