[relay] add missing virtual d'tor

Add a default virtual destructor to
`tvm::relay::transforms::GlobalSymbolCache`, so that
correct destructors run when destroying
subclass instances.

Author: Christian Convey

src/relay/transforms/compiler_function_utils.cc

@@ -119,6 +119,8 @@ class CallRewriter : public MixedModeMutator {
 
 }  // namespace
 
+GlobalSymbolCache::~GlobalSymbolCache() = default;
+
 GlobalVar ExistingGlobalSymbolCache::GetGlobalSymbol(const Function& function) {
   Optional<String> opt_global_symbol = function->GetAttr<String>(tvm::attr::kGlobalSymbol);
   ICHECK(opt_global_symbol.defined())

src/relay/transforms/compiler_function_utils.h

@@ -71,6 +71,7 @@ namespace transforms {
  */
 class GlobalSymbolCache {
  public:
+  virtual ~GlobalSymbolCache();
   virtual GlobalVar GetGlobalSymbol(const Function& function) = 0;
 };