Fix PC Tools usage of legacy xp_mysql.dll plugin, fix build error.

- The PC Tools script used the stock NWNX SQL library and was
  still going to xp_mysql.dll in some circumstances as a result.

  Modify our local nwnx_sql.nss to simply use the proper ACR
  persist functionality instead.

- Also, fix pre-existing build break in acr_xp_i.nss.