patch 8.1.0644: finding next sign ID is inefficient

Problem:    Finding next sign ID is inefficient.
Solution:   Add next_sign_id. (Yegappan Lakshmanan, closes #3717)

src/structs.h

@@ -737,6 +737,7 @@ typedef struct proptype_S
 typedef struct signgroup_S
 {
     short_u	refcount;		// number of signs in this group
+    int		next_sign_id;		// next sign id for this group
     char_u	sg_name[1];		// sign group name
 } signgroup_T;