Fix wrong import.

Before the program would fail with:

```
  File ".../ledgerhelpers/src/ledgerhelpers/programs/withdrawcli.py", line 45, in main
    amount1 = common.prompt_for_amount(
AttributeError: module 'ledgerhelpers.legacy' has no attribute 'prompt_for_amount'. Did you mean: 'prompt_for_account'?
```

src/ledgerhelpers/programs/withdrawcli.py

@@ -6,6 +6,7 @@
 import sys
 import ledgerhelpers
 import ledgerhelpers.legacy as common
+import ledgerhelpers.legacy_needsledger as common2
 import ledgerhelpers.journal as journal
 
 
@@ -42,12 +43,12 @@ def main():
     s["last_deposit_account"] = asset2
     asset2_currency = commodities.get(asset2, ledger.Amount("$ 1"))
 
-    amount1 = common.prompt_for_amount(
+    amount1 = common2.prompt_for_amount(
         sys.stdin, sys.stdout,
         "How much?", asset1_currency
     )
 
-    amount2 = common.prompt_for_amount(
+    amount2 = common2.prompt_for_amount(
         sys.stdin, sys.stdout,
         "What was deposited?", asset2_currency
     )