patch 8.1.2373: cannot build with +popupwin but without +quickfix

Problem:    Cannot build with +popupwin but without +quickfix. (John Marriott)
Solution:   Adjust #ifdefs.
diff --git a/src/testdir/test_preview.vim b/src/testdir/test_preview.vim
index 91923fb..a85798b 100644
--- a/src/testdir/test_preview.vim
+++ b/src/testdir/test_preview.vim
@@ -1,4 +1,6 @@
 " Tests for the preview window
+source check.vim
+CheckFeature quickfix
 
 func Test_Psearch()
   " this used to cause ml_get errors