BUILD: fix build warning when not using 51degrees

An ifdef was missing to avoid declaring these variables :

src/haproxy.c: In function 'deinit':
src/haproxy.c:1253:47: warning: unused variable '_51d_prop_nameb' [-Wunused-variable]
src/haproxy.c:1253:30: warning: unused variable '_51d_prop_name' [-Wunused-variable]

src/haproxy.c

@@ -1250,7 +1250,9 @@ void deinit(void)
 	struct logsrv *log, *logb;
 	struct logformat_node *lf, *lfb;
 	struct bind_conf *bind_conf, *bind_back;
+#ifdef USE_51DEGREES
 	struct _51d_property_names *_51d_prop_name, *_51d_prop_nameb;
+#endif
 	int i;
 
 	deinit_signals();