D4896 added the function GetVirtualTransactionSize that references
::nBytesPerSigOp, a global initialized by the constant
DEFAULT_BYTES_PER_SIGOP which is defined in policy.h
This was naively 'fixed' in D5967 by me including policy/settings.h in
policy.h which was wrong, and will be (is) a problem when backporting
PR15639, which includes policy/policy.h in policy/settings.h, creating
an include loop that the compiler does not like.
This diff corrects the issue by actually referencing DEFAULT_BYTES_PER_SIGOP
instead of doing it the roundabout way.
That let us remove #include <policy/settings.h> from policy/policy.h,
which revealed that wallet/wallet.cpp was depending on it being indirectly
included for dustRelayFee. So #include <policy/settings.h> is added
to wallet/wallet.cpp