patch 8.1.1881: popup window test fails in some configurations

Problem:    Popup window test fails in some configurations.
Solution:   Check that screendumps can be made.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 5de7255..07ed81b 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -2194,6 +2194,8 @@
 endfunc
 
 func Test_popupmenu_info()
+  CheckScreendump
+
   let lines =<< trim END
       set completeopt+=preview,popup
       set completefunc=CompleteFuncDict
diff --git a/src/version.c b/src/version.c
index 0611690..15b77c3 100644
--- a/src/version.c
+++ b/src/version.c
@@ -770,6 +770,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1881,
+/**/
     1880,
 /**/
     1879,