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/main.c b/src/main.c
index d24eafa..2f8f052 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1032,6 +1032,10 @@
 #ifdef FEAT_EVAL
     set_lang_var();		/* set v:lang and v:ctype */
 #endif
+
+#ifdef FEAT_SIGNS
+    init_signs();
+#endif
 }
 
 /*