Fix missing @clicksequence problem for building Docbook output also.

Commit 770f21f5 "Fix missing @clicksequence problem with older Texinfo
versions." fixed the problem of missing @clicksequence for building the
main PSPP documentation, partly by adding to $(AM_MAKEINFOFLAGS).  However,
$(AM_MAKEINFOFLAGS) wasn't used in building the Docbook output for use by
Yelp as the GUI help.  This commit adds $(AM_MAKEINFOFLAGS) to that
command, fixing the problem.