sd-bus: silence format warnings in kdbus code (#6072)

The code is mostly correct, but gcc is trying to outsmart us, and emits a
warning for a "llu vs lu" mismatch, even though they are the same size (on alpha):

src/libsystemd/sd-bus/bus-control.c: In function ‘kernel_get_list’:
src/libsystemd/sd-bus/bus-control.c:267:42: error: format ‘%llu’ expects argument of type ‘long long unsigned int’, but argument 3 has type ‘__u64 {aka long unsigned int}’ [-Werror=format=]
                         if (asprintf(&n, ":1.%llu", name->id) < 0) {
                                          ^
src/libsystemd/sd-bus/bus-control.c: In function ‘bus_get_name_creds_kdbus’:
src/libsystemd/sd-bus/bus-control.c:714:47: error: format ‘%llu’ expects argument of type ‘long long unsigned int’, but argument 3 has type ‘__u64 {aka long unsigned int}’ [-Werror=format=]
                 if (asprintf(&c->unique_name, ":1.%llu", conn_info->id) < 0) {
                                               ^
This is hard to work around properly, because kdbus.h uses __u64 which is
defined-differently-despite-being-the-same-size then uint64_t. Thus the simple
solution of using %PRIu64 fails on amd64:

src/libsystemd/sd-bus/bus-control.c:714:47: error: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 3 has type ‘__u64 {aka long long unsigned int}’ [-Werror=format=]
                 if (asprintf(&c->unique_name, ":1.%"PRIu64, conn_info->id) < 0) {
                                               ^~~~~~

Let's just avoid the whole issue for now by silencing the warning.
After the next release, we should just get rid of the kdbus code.

Fixes #5561.