initialize base class in copy constructors (#2006)

GCC issues warnings with -Wextra if we don't explicitly initialize
base class in copy constructors. This commit fixed the issue.

include/tvm/runtime/packed_func.h

@@ -603,7 +603,7 @@ class TVMRetValue : public TVMPODValue_ {
   using TVMPODValue_::operator TVMContext;
   using TVMPODValue_::operator NDArray;
   // Disable copy and assign from another value, but allow move.
-  TVMRetValue(const TVMRetValue& other) {
+  TVMRetValue(const TVMRetValue& other) : TVMPODValue_() {
     this->Assign(other);
   }
   // conversion operators