patch 8.1.1667: flags for Ex commands may clash with other symbols

Problem:    Flags for Ex commands may clash with other symbols.
Solution:   Prepend with EX_.
diff --git a/src/version.c b/src/version.c
index f274cef..afd775a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1667,
+/**/
     1666,
 /**/
     1665,