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)
diff --git a/src/globals.h b/src/globals.h
index 0a29511..71400ca 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -609,7 +609,7 @@
 #define FOR_ALL_BUFFERS(buf) for (buf = firstbuf; buf != NULL; buf = buf->b_next)
 
 // Iterate through all the signs placed in a buffer
-#define FOR_ALL_SIGNS_IN_BUF(buf) \
+#define FOR_ALL_SIGNS_IN_BUF(buf, sign) \
 	for (sign = buf->b_signlist; sign != NULL; sign = sign->next)
 
 /* Flag that is set when switching off 'swapfile'.  It means that all blocks