varlink-idl: allow to pass polkit arguments to io.systemd.service.Reload()

Then, varlink interfaces with unprivileged access can be implement the
method.

src/shared/varlink-io.systemd.service.c

@@ -2,6 +2,7 @@
 
 #include <unistd.h>
 
+#include "bus-polkit.h"
 #include "env-util.h"
 #include "json-util.h"
 #include "log.h"
@@ -11,7 +12,9 @@
 
 static SD_VARLINK_DEFINE_METHOD(Ping);
 
-static SD_VARLINK_DEFINE_METHOD(Reload);
+static SD_VARLINK_DEFINE_METHOD(
+                Reload,
+                VARLINK_DEFINE_POLKIT_INPUT);
 
 static SD_VARLINK_DEFINE_METHOD(
                 SetLogLevel,