proc_each_library should iterate from passed-in iterator non-inclusive

- the semantics should be start_after, as in other cases in ltrace
diff --git a/proc.c b/proc.c
index acd37e8..b1c0a94 100644
--- a/proc.c
+++ b/proc.c
@@ -908,6 +908,8 @@
 {
 	if (it == NULL)
 		it = proc->libraries;
+	else
+		it = it->next;
 
 	while (it != NULL) {
 		struct library *next = it->next;