dir: fix build when HAVE_LIBSYSTEMD but not USE_SYSTEM_HELPER

polkit_subject_to_string() is called inside the HAVE_LIBSYSTEMD guard
in flatpak_dir_log(), but <polkit/polkit.h> is only included when
USE_SYSTEM_HELPER is defined. This causes a build failure on
configurations that have libsystemd but no system helper.

Guard the polkit call with USE_SYSTEM_HELPER and fall back to "(none)"
so the subject string is always valid for the sd_journal_send() call.

Fixes: f9d5c5c ("dir: Free result of polkit_subject_to_string")
Signed-off-by: Rudi Heitbaum <rudi@heitbaum.com>

common/flatpak-dir.c

@@ -17463,7 +17463,11 @@ static void
   va_list args;
 
   installation = source ? source : flatpak_dir_get_name_cached (self);
+#ifdef USE_SYSTEM_HELPER
   subject = self->subject ? polkit_subject_to_string (self->subject) : g_strdup ("(none)");
+#else
+  subject = g_strdup ("(none)");
+#endif
 
   len = g_snprintf (message, sizeof (message), "%s: ", installation);