patch 8.1.2405: matchadd_conceal test fails under valgrind

Problem:    matchadd_conceal test fails under valgrind.
Solution:   Use WaitForAssert() and wait a bit longer.
diff --git a/src/version.c b/src/version.c
index 53b9538..89896e5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -743,6 +743,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2405,
+/**/
     2404,
 /**/
     2403,