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.

lib/layouts/sigplanconf.layout

@@ -120,6 +120,13 @@ Style PreprintFooter
 End
 
 
+Style DOI
+	CopyStyle	CopyrightYear
+	LatexName	doi
+	LabelString	"Digital Object Identifier:"
+End
+
+
 Input stdtitle.inc
 Input stdstruct.inc
 

lib/templates/ACM-sigplan.lyx

@@ -147,6 +147,20 @@ is ignored unless document class option 'preprint' is used
 \end_inset
 
 
+\end_layout
+
+\begin_layout DOI
+10.1145/143365.143376
+\begin_inset Note Note
+status collapsed
+
+\begin_layout Plain Layout
+the digital identifier assigned at doi.acm.org
+\end_layout
+
+\end_inset
+
+
 \end_layout
 
 \begin_layout Title