patch 8.1.0881: can execute shell commands in rvim through interfaces

Problem:    Can execute shell commands in rvim through interfaces.
Solution:   Disable using interfaces in restricted mode. Allow for writing
            file with writefile(), histadd() and a few others.
diff --git a/src/version.c b/src/version.c
index 1235cf0..48236fe 100644
--- a/src/version.c
+++ b/src/version.c
@@ -784,6 +784,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    881,
+/**/
     880,
 /**/
     879,