cmd/goyacc: add header to generated go output file
cmd/goyacc: add header to generated go output file

Add "// Code generated by goyacc. DO NOT EDIT." to the top of the
generated go output file.

Fixes golang/go#26158

Change-Id: I5dd66af9cd8ccccd27440886c55cd604686472e3
Reviewed-on: https://go-review.googlesource.com/122095
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools