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
[[https://github.com/bitcoin/bitcoin/pull/15639 | 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