vcl: Fix vcldemo linkage - was missing tk

Change-Id: I74d395c0fdcbdb5117584af329e6a023414d46af
Reviewed-on: https://gerrit.libreoffice.org/11806
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>