Fixed build failure with PFS disabled

mariabackup fails to builds with PFS disabled. The reason was missing include,
which was included by PFS otherwise.

extra/mariabackup/common.h

@@ -25,6 +25,7 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 #include <mysql_version.h>
 #include <fcntl.h>
 #include <stdarg.h>
+#include <my_sys.h>
 
 
 # define fil_is_user_tablespace_id(i) ((i) > srv_undo_tablespaces_open)