LaTeX template: set secnumdepth to 5 if --number-sections specified.

This yields behavior equivalent to the other writers, numbering
level 4 and 5 headers too.

default.latex

@@ -102,6 +102,7 @@ $endif$
 \setlength{\parskip}{6pt plus 2pt minus 1pt}
 \setlength{\emergencystretch}{3em}  % prevent overfull lines
 $if(numbersections)$
+\setcounter{secnumdepth}{5}
 $else$
 \setcounter{secnumdepth}{0}
 $endif$