sd-bus: export sd_bus_call{,_async,_async_cancel}

The .sym file somehow lacks these declarations, so add these. You have to
run "make clean" to make sure the sym-test runs fine afterwards.

src/libsystemd/libsystemd.sym

@@ -112,6 +112,9 @@ global:
         sd_bus_get_name;
         sd_bus_send;
         sd_bus_send_to;
+        sd_bus_call;
+        sd_bus_call_async;
+        sd_bus_call_async_cancel;
         sd_bus_get_fd;
         sd_bus_get_events;
         sd_bus_get_timeout;