recently backported change conflicts with D753 by forward declaring
CMutableTransaction as a struct, not a class.
following warning issued:
../src/./rpc/rawtransaction_util.h:12:1: warning: struct 'CMutableTransaction' was previously declared as a class; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Wmismatched-tags] struct CMutableTransaction;
this changes the forward declaration to class for consistency