Fix format string mismatch introduced in ab9001a1

src/libsystemd-bus/sd-bus.c: In function 'sd_bus_open_user':
src/libsystemd-bus/sd-bus.c:1104:25: warning: format '%s' expects argument of type 'char *', but argument 3 has type 'long unsigned int' [-Wformat=]
                         asprintf(&b->address, UNIX_USER_BUS_FMT, (unsigned long) getuid());