Add possibility to create hardlinks.

Leads to much faster brp-25-symlink for building mixxx on PMBS:
-[  761s] calling /usr/lib/rpm/brp-suse.d/brp-25-symlink
-[ 2080s] calling /usr/lib/rpm/brp-suse.d/brp-30-desktop
+[ 1510s] calling /usr/lib/rpm/brp-suse.d/brp-25-symlink
+[ 1511s] calling /usr/lib/rpm/brp-suse.d/brp-30-desktop