patch 8.1.0378: CI build failure

Problem:    CI build failure.
Solution:   Include vim.h as ../vim.h.  Fix compiler warning.
diff --git a/src/xdiff/xdiff.h b/src/xdiff/xdiff.h
index 476b20c..a8bc1dc 100644
--- a/src/xdiff/xdiff.h
+++ b/src/xdiff/xdiff.h
@@ -108,7 +108,7 @@
 	long bsize;
 } bdiffparam_t;
 
-#include "vim.h"
+#include "../vim.h"
 
 #define xdl_malloc(x) lalloc((x), TRUE)
 #define xdl_free(ptr) vim_free(ptr)