BUG/MINOR: server: do not report diag for peer servers with null weight

Only check servers attached to a proxy with PR_CAP_LB.

This does not need to be backported as the diag message was added in the
current 2.4-dev branch.
diff --git a/src/server.c b/src/server.c
index 9b83ff8..75977d8 100644
--- a/src/server.c
+++ b/src/server.c
@@ -2703,7 +2703,7 @@
 	if (parse_flags & SRV_PARSE_TEMPLATE)
 		_srv_parse_tmpl_init(newsrv, curproxy);
 
-	HA_DIAG_WARNING_COND(!newsrv->uweight,
+	HA_DIAG_WARNING_COND((curproxy->cap & PR_CAP_LB) && !newsrv->uweight,
 	                     "parsing [%s:%d] : 'server %s' : configured with weight of 0 will never be selected by load balancing algorithms\n",
 	                     file, linenum, newsrv->id);