Fix compilation of the template ACM-sigplan.lyx

Fix compilation of the template ACM-sigplan.lyx

This template was only reported as failing to compile by LyX as of
0a6120cb. I do not know how long this template was actually failing
to compile without error. The problem was that the class requires
\doi to be defined. This might not have been a requirement in
previous versions of the class.

Thanks to Enrico for finding the problem and solution.