Revert "sd-bus: use PRIu64 instead of casting" (#4556)

This reverts commit 75ead2b7.

Follow up for #4546:
> @@ -848,8 +848,7 @@ static int bus_kernel_make_message(sd_bus *bus, struct kdbus_msg *k) {
         if (k->src_id == KDBUS_SRC_ID_KERNEL)
                 bus_message_set_sender_driver(bus, m);
         else {
-                xsprintf(m->sender_buffer, ":1.%llu",
-                         (unsigned long long)k->src_id);
+                xsprintf(m->sender_buffer, ":1.%"PRIu64, k->src_id);

This produces:

src/libsystemd/sd-bus/bus-kernel.c: In function ‘bus_kernel_make_message’:
src/libsystemd/sd-bus/bus-kernel.c:851:44: warning: format ‘%lu’ expects argument of type ‘long
unsigned int’, but argument 4 has type ‘__u64 {aka long long unsigned int}’ [-Wformat=]
                 xsprintf(m->sender_buffer, ":1.%"PRIu64, k->src_id);
                                            ^