Added using statement to make compile with command

[icyfire0573]

dotnet build OpenXmlPowerTools.sln

apparently notepad isn't the way to edit this file as it shows everythign changed but I just added

using System.Diagnostics;

as line 3 or it wouldn't compile for me.

[msftclas]

All CLA requirements met.

[flensrocker]

Same as #266

I chose to add the namespace directly on the lines where they are needed, so there isn't an "unused using" at the top of the file

[twsouthwick]

Thanks for the contribution. I merged the other one before I saw your change. Feel free to merge those changes and update yours if you prefer.