src: ignore GCC -Wcast-function-type for v8.h

Currently, the following warnings are emitted during compilation:
In file included from ../src/string_bytes.h:29,
                 from ../src/string_bytes.cc:22:
../deps/v8/include/v8.h:
In instantiation of ‘void v8::PersistentBase<T>::SetWeak(
    P*,typename v8::WeakCallbackInfo<P>::Callback, v8::WeakCallbackType)
[with P = node::BaseObject;
T = v8::Object;
typename v8::WeakCallbackInfo<P>::Callback =
    void (*)(const v8::WeakCallbackInfo<node::BaseObject>&)]’:
../src/base_object-inl.h:135:42:   required from here
../deps/v8/include/v8.h:10732:16: warning:
cast between incompatible function types from
‘v8::WeakCallbackInfo<node::BaseObject>::Callback’ {aka
‘void (*)(const v8::WeakCallbackInfo<node::BaseObject>&)’} to
‘Callback’ {aka ‘void (*)(const v8::WeakCallbackInfo<void>&)’}
[-Wcast-function-type]
10732 |                reinterpret_cast<Callback>(callback), type);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

And the...