Dev builds break previously working plugin (Permissions Manager)

[PaulFromNZ]

The last few dev and dev commit builds (can't determine how far back this started) have broken Permissions Manager By Steenamaroo (Hosted on Codefling)
It now fails to compile with the following error:

Failed compiling 'C:\GAMES\RUST\carbon\plugins\PermissionsManager.cs':

1. The type 'DbConnection' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Data, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
   (PermissionsManager 29 line 1032)

Line 1032 reads: try { Sql_conn?.Con?.Open(); }