Added Bricx Command Center gitignore

Added gitignore for Bricx Command Center (BricxCC) IDE – http://bricxcc.sourceforge.net

BricxCC always generates a `.bak` and `.sym` file for every program - they are just backup files and should be ignored.

You might say that BricxCC is a bit niche, but it's widely used throughout the (Lego) robotics industry.