Fix -Wdeprecated-copy warning with GCC >= 9
Summary:
../src/psbt.cpp: In function 'bool CombinePSBTs(PartiallySignedTransaction&, TransactionError&, const std::vector<PartiallySignedTransaction>&)':
../src/psbt.cpp:214:19: warning: implicitly-declared 'PartiallySignedTransaction& PartiallySignedTransaction::operator=(const PartiallySignedTransaction&)' is deprecated [-Wdeprecated-copy]
214 | out = psbtxs[0];
| ^
In file included from ../src/psbt.cpp:5:
../src/./psbt.h:327:5: note: because 'PartiallySignedTransaction' has user-provided 'PartiallySignedTransaction::PartiallySignedTransaction(const PartiallySignedTransaction&)'
327 | PartiallySignedTransaction(const PartiallySignedTransaction &psbt_in)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~Test Plan:
With GCC >= 9:
ninja all check
Check the warning is gone.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D5862