Import Data.ByteString.Char8 to make it compile on Debian 7

The error message was:
````
src/Network/Protocol/NetSNMP.hsc:505:27:
    No instance for (Data.String.IsString ByteString)
      arising from the literal `"."'
    Possible fix:
      add an instance declaration for (Data.String.IsString ByteString)
    In the first argument of `B.intercalate', namely `"."'
    In the expression: B.intercalate "." (map (B.pack . (: [])) octets)
    In an equation for `str':
        str = B.intercalate "." (map (B.pack . (: [])) octets)
````