DiscIO/VolumeVerifier: Default destructor within the cpp file

Given the volume verifier has quite a few non-trivial object within it,
it's best to default the destructor within the cpp file to prevent
inlining complex destruction logic elsewhere, while also making it nicer
if a forward-declared type is ever used in a member variable.

Source/Core/DiscIO/VolumeVerifier.cpp

@@ -54,6 +54,8 @@ VolumeVerifier::VolumeVerifier(const Volume& volume, Hashes<bool> hashes_to_calc
 {
 }
 
+VolumeVerifier::~VolumeVerifier() = default;
+
 void VolumeVerifier::Start()
 {
   ASSERT(!m_started);