Update MessageBuilder.h - add function didn't take size_t parameter

The add function didn't have explicit support for the size_t argument type.

This resulted in:

thehub/libs/apputils/Blockchain.cpp:915:44: error: call of overloaded ‘add(Blockchain::HeaderTags, size_t&)’ is ambiguous
                 builder.add(JobRequestId, i);
                                            ^