patch 8.1.1391: no popup window support

Problem:    No popup window support.
Solution:   Add initial code for popup windows.  Add the 'wincolor' option.
diff --git a/src/proto.h b/src/proto.h
index 4d2eded..e606f08 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -195,6 +195,7 @@
 #  include "termlib.pro"
 # endif
 # ifdef FEAT_TEXT_PROP
+#  include "popupwin.pro"
 #  include "textprop.pro"
 # endif
 # include "ui.pro"