Restore 'xsltproc' discovery for statistics system test 9.16

xsltproc is used by the statistics system test and its discovery was removed in the transition to sphinx for documents.

I:statistics:checking bind9.xsl vs xml (16)
I:statistics:skipping test as libxml2 and/or curl and/or xsltproc was not found
I:statistics:checking bind9.xml socket statistics (17)
I:statistics:skipping test as libxml2 and/or curl and/or xsltproc was not found
I:statistics:checking priming queries are counted (18)
I:statistics:Check that 'zone-statistics full;' is processed by 'rndc reconfig' (19)
I:statistics:exit status: 0
I:statistics:stopping servers