cgen: Add .gitignore
cgen: Add .gitignore

Used to ignore locally-generated output files in cgen.

Fixes: 9e6b83e5 ("Single Source of Truth.")

karolherbst/envytools,pierremoreau/envytools,karolherbst/envytools,pierremoreau/envytools,karolherbst/envytools,envytools/envytools,pierremoreau/envytools,envytools/envytools,envytools/envytools,envytools/envytools,karolherbst/envytools,karolherbst/envytools,envytools/envytools,pierremoreau/envytools,pierremoreau/envytools