Fix a compilation failure if yajl not avail

configure:     yajl: no
CC     libvirt_util_la-json.lo
util/json.c:32:27: error: yajl/yajl_gen.h: No such file or directory
util/json.c:33:29: error: yajl/yajl_parse.h: No such file or directory

* src/util/json.c: remove the includes if yajl not configured in

src/util/json.c

@@ -29,8 +29,10 @@
 #include "logging.h"
 #include "util.h"
 
+#if HAVE_YAJL
 #include <yajl/yajl_gen.h>
 #include <yajl/yajl_parse.h>
+#endif
 
 /* XXX fixme */
 #define VIR_FROM_THIS VIR_FROM_NONE