userdb: fix dlopen call

The call would always fail with:
systemd-userwork[780]: Failed to dlopen(libnss_systemd.so.2), ignoring: /usr/lib64libnss_systemd.so.2: cannot open shared object file: No such file or directory

Author: keszybz

src/shared/userdb.c

@@ -1231,7 +1231,7 @@ int userdb_block_nss_systemd(int b) {
 
         /* Note that we might be called from libnss_systemd.so.2 itself, but that should be fine, really. */
 
-        dl = dlopen(ROOTLIBDIR "libnss_systemd.so.2", RTLD_LAZY|RTLD_NODELETE);
+        dl = dlopen(ROOTLIBDIR "/libnss_systemd.so.2", RTLD_LAZY|RTLD_NODELETE);
         if (!dl) {
                 /* If the file isn't installed, don't complain loudly */
                 log_debug("Failed to dlopen(libnss_systemd.so.2), ignoring: %s", dlerror());