patch 8.1.0556: saving/restoring search patterns share saved last_idx

Problem:    Saving/restoring search patterns share saved last_idx.
Solution:   Use a separate saved last_idx for saving search patterns for
            functions and incremental search.
diff --git a/src/version.c b/src/version.c
index 566f94d..55a9af6 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    556,
+/**/
     555,
 /**/
     554,