debian/control: depend on php-xml instead of php-xsl

Don't depend on php-xsl which is a virtual package, nowadays depending
on php-xml is the right thing to do.

This also fixes a warning from cme:

  Warning in 'control binary:tweeper Depends:3' value 'php-xsl': \
  package php-xsl is unknown. Check for typos if not a virtual package.