as those are fake, just textfiles with dll extension that signtool
doesn't like.
Also made the text more descriptive "invalid" is ambiguous,
"invalid - merged lib" states the reason why the file is a dummy.

Change-Id: I31801fd0c3aa593549fac5e6275189e18bbc0010
Reviewed-on: https://gerrit.libreoffice.org/15444Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Jenkins <ci@libreoffice.org>