MDEV-20844: Add missing override qualifiers

clang warns if only some overridden member functions in a class
are tagged with the C++11 override keyword. Add the missing keywords.

plugin/type_inet/sql_type_inet.cc

@@ -984,12 +984,12 @@ class Field_inet6: public Field
              unpack(to, from, from_end, param_data);
   }
 
-  uint max_packed_col_length(uint max_length)
+  uint max_packed_col_length(uint max_length) override
   {
     return StringPack::max_packed_col_length(max_length);
   }
 
-  uint packed_col_length(const uchar *data_ptr, uint length)
+  uint packed_col_length(const uchar *data_ptr, uint length) override
   {
     return StringPack::packed_col_length(data_ptr, length);
   }