Avoid lintian error

  qhull-bin: manpage-has-errors-from-man

to whit

  usr/share/man/man1/qhull.1.gz 750: warning: macro `QJn'' not defined
  usr/share/man/man1/qhull.1.gz 771: warning: macro `Qt'' not defined

and consequent lost text.  These result from a line starting with the
single quote character; this patch backslash escapes them.