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)
Merge pull request #2 from KaiHa/master

Import the IsString instance from Data.ByteString.Char8 to make it compile on Debian 7