[relay] Fix missing virtual destructor

Add missing virtual destructor for derived class
`tvm::relay::transforms::ExistingGlobalSymbolCache`.
Fixes a potential problem when deleting an instance
using a pointer-to-base-class.

Author: Christian Convey

src/relay/transforms/compiler_function_utils.cc

@@ -119,6 +119,8 @@ class CallRewriter : public MixedModeMutator {
 
 }  // namespace
 
+ExistingGlobalSymbolCache::~ExistingGlobalSymbolCache() {}
+
 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

@@ -80,6 +80,8 @@ class GlobalSymbolCache {
  */
 class ExistingGlobalSymbolCache : public GlobalSymbolCache {
  public:
+  virtual ~ExistingGlobalSymbolCache();
+
   ExistingGlobalSymbolCache() = default;
 
   GlobalVar GetGlobalSymbol(const Function& function) final;