build: fix build when not using dbus

Commit c0e70221 breaks on a machine that lacks dbus headers:

In file included from util/virdbus.c:24:0:
util/virdbuspriv.h:31:3: error: unknown type name 'dbus_int16_t'

* src/util/virdbuspriv.h (DBusBasicValue): Only provide fallback
when dbus is compiled.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/util/virdbuspriv.h

@@ -24,7 +24,7 @@
 
 # include "virdbus.h"
 
-# if !HAVE_DBUSBASICVALUE
+# if defined(WITH_DBUS) && !HAVE_DBUSBASICVALUE
 /* Copied (and simplified) from dbus 1.6.12, for use with older dbus headers */
 typedef union
 {