coredump: fix one memleak in backtrace mode

Journal importer internal structures need to be freed.

src/coredump/coredump.c

@@ -1270,7 +1270,7 @@ static int process_backtrace(int argc, char *argv[]) {
         _cleanup_free_ struct iovec *iovec = NULL;
         size_t n_iovec, n_allocated, n_to_free = 0, i;
         int r;
-        JournalImporter importer = {
+         _cleanup_(journal_importer_cleanup) JournalImporter importer = {
                 .fd = STDIN_FILENO,
         };