patch 8.1.0088: terminal test for stdout and stderr is a bit flaky

Problem:    Terminal test for stdout and stderr is a bit flaky.
Solution:   Wait for both stdout and stderr to have been processed. (Ozaki
            Kiichi, closes #2991)
diff --git a/src/version.c b/src/version.c
index 72a8238..9a1fbc6 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    88,
+/**/
     87,
 /**/
     86,