patch 8.1.0135: undo message delays screen update for CTRL-O u

Problem:    Undo message delays screen update for CTRL-O u.
Solution:   Add smsg_attr_keep(). (closes #3125)
diff --git a/src/proto.h b/src/proto.h
index 655b72c..8570af9 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -119,6 +119,12 @@
 #  ifdef __BORLANDC__
 _RTLENTRYF
 #  endif
+smsg_attr_keep(int, char_u *, ...);
+
+int
+#  ifdef __BORLANDC__
+_RTLENTRYF
+#  endif
 vim_snprintf_add(char *, size_t, char *, ...)
 #ifdef USE_PRINTF_FORMAT_ATTRIBUTE
     __attribute__((format(printf, 3, 4)))