pallet-xcm: fix transport fees for remote reserve transfers (#3798)

Add `SetFeesMode { jit_withdraw: true }` before
`InitiateReserveWithdraw` instruction to allow paying JIT fees in the
executor.

This is backport of #3792

polkadot/xcm/pallet-xcm/src/lib.rs

@@ -1883,6 +1883,7 @@ impl<T: Config> Pallet<T> {
 		]);
 		Ok(Xcm(vec![
 			WithdrawAsset(assets.into()),
+			SetFeesMode { jit_withdraw: true },
 			InitiateReserveWithdraw {
 				assets: Wild(AllCounted(max_assets)),
 				reserve,