MINOR: cfgparse: Add a cast to make gcc happier.

When calling calloc(), cast global.nbthread to unsigned int, so that gcc
doesn't freak out, as it has no way of knowing global.nbthread can't be
negative.
diff --git a/src/cfgparse.c b/src/cfgparse.c
index dcbcb48..4c8b1fe 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -3596,7 +3596,7 @@
 						LIST_INIT(&toremove_connections[i]);
 					}
 				}
-				newsrv->idle_orphan_conns = calloc(global.nbthread, sizeof(*newsrv->idle_orphan_conns));
+				newsrv->idle_orphan_conns = calloc((unsigned int)global.nbthread, sizeof(*newsrv->idle_orphan_conns));
 				if (!newsrv->idle_orphan_conns)
 					goto err;
 				for (i = 0; i < global.nbthread; i++)