Fix struct vs class mismatch for Amount

Summary:
The recently introduced (D3344 and D3365) forward declarations for
class Amount conflict with the declared struct Amount from amount.h.
This throws -Wmismatched-tags warnings.

Test Plan:

make check
ninja check

Make sure there is no -Wmismatched-tags warning

Reviewers: #bitcoin_abc, deadalnix, jasonbcox, markblundeberg

Reviewed By: #bitcoin_abc, jasonbcox, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3419