MDEV-30953: Add MariaDB-server-galera (RPM) fix

[grooverdan]

The wsrep SST scripts weren't part of the server-galera component so CPack didn't put them in the MariaDB-server-galera package.

The previous existing per file component wasn't used so this was simplified.

Noticed thanks to @FaramosCZ in #4871.

$ cmake --install . --prefix /tmp/sd --component server-galera
-- Install configuration: "RelWithDebInfo"
-- Installing: /tmp/sd/lib/plugin/wsrep_info.so
-- Installing: /tmp/sd/bin/wsrep_sst_rsync_wan
-- Installing: /tmp/sd/bin/wsrep_sst_mysqldump
-- Installing: /tmp/sd/bin/wsrep_sst_rsync
-- Installing: /tmp/sd/bin/wsrep_sst_mariabackup
-- Installing: /tmp/sd/bin/wsrep_sst_backup
-- Installing: /tmp/sd/bin/wsrep_sst_common
-- Installing: /tmp/sd/support-files/wsrep.cnf
-- Installing: /tmp/sd/support-files/mariadb.service.d/galera.conf
-- Installing: /tmp/sd/support-files/systemd/use_galera_new_cluster.conf
(base) 
~/repos/build-mariadb-server-12.3 
$ ls -la  /tmp/sd/bin/
total 172
drwxr-xr-x. 2 dan dan   160 Apr 10 11:21 .
drwxr-xr-x. 5 dan dan   100 Apr 10 11:21 ..
-rwxr-xr-x. 1 dan dan  2451 Mar 27 15:24 wsrep_sst_backup
-rw-r--r--. 1 dan dan 69888 Mar 27 15:24 wsrep_sst_common
-rwxr-xr-x. 1 dan dan 53228 Mar 27 15:24 wsrep_sst_mariabackup
-rwxr-xr-x. 1 dan dan  9035 Mar 27 15:24 wsrep_sst_mysqldump
-rwxr-xr-x. 1 dan dan 30564 Mar 27 15:24 wsrep_sst_rsync
lrwxrwxrwx. 1 dan dan    15 Apr 10 11:21 wsrep_sst_rsync_wan -> wsrep_sst_rsync
```