patch 8.1.0504: when CTRL-C is mapped it triggers InsertLeave

Problem:    When CTRL-C is mapped it triggers InsertLeave.
Solution:   Make CTRL-C behave the same way when typed or used in a mapping.
diff --git a/src/version.c b/src/version.c
index bc22e73..2c06a0f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    504,
+/**/
     503,
 /**/
     502,