correct forward declaration in rawtransaction_util.h

Summary:
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

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5993