GNU xargs executes the command at least once even if the standard input
is empty, unlike BSD xargs, which causes rm -r to be called with no
arguments ween the find command finds nothing leading to an error.
Adding -f to rm allows buikding with either implementation.

Change-Id: I0df5fcb379d2a5a8b1121594ec1a82d917d80dfc
Reviewed-on: https://gerrit.libreoffice.org/16116Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: David Tardon <dtardon@redhat.com>