[Fixed] Function 'Screen.GetTextScreenSize' not exported

[lemming]

Commit SHA: a8a5613675e1760e079acc6a3992c6e362601cdb

On launch, regardless of IWAD selection, I'm getting the following error on startup:

gzdoom.pk3:zscript/engine/base.zs, line 415: The function 'Screen.GetTextScreenSize' has not been exported from the executable

[Graf Zahl]

This was accidentally copied over from Raze. The declaration had to be removed.