spambayes: Add bsddb build input

To reproduce the issue:
```
nix-build -E 'import <nixpkgs> {}' -A python27Packages.spambayes
result/bin/sb_filter.py </dev/null
```
Before this commit, this fails with
```
[...]
  File "/nix/store/vlgr1wrxd2wiw5ay5ch54365dzqvcwgq-python2.7-spambayes-1.1b1/lib/python2.7/site-packages/spambayes/dbmstorage.py", line 20, in open_dbhash
    return bsddb.hashopen(*args)
AttributeError: 'NoneType' object has no attribute 'hashopen'
```
After this commit, it doesn't.