patch 8.1.0055: complete test has wrong order of arguments

Problem:    Complete test has wrong order of arguments. Wrong type for
            sentinel variable.
Solution:   Swap arguments, use VAR_UNKNOWN. (Ozaki Kiichi)
diff --git a/src/version.c b/src/version.c
index 90ec0e4..8d58649 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    55,
+/**/
     54,
 /**/
     53,