Update tpcc-sqlserver-ddl.sql

Changed 'smalldatetime' to 'datetime'.

'smalldatetime' is an unfair optimization for MS SQL from my point of view.
But the main reason is the incompatibility with the CHBenchmark, which does in Query 10 this:
"AND o_entry_d >= '2007-01-02 00:00:00.000000'"

This line fails in MSSQL with: "Fehler beim Konvertieren einer Zeichenfolge in den smalldatetime-Datentyp" (german, I think the corresponding English error is "Conversion failed when converting character string to smalldatetime data type.")