patch 8.1.0579: cannot attach properties to text

Problem:    Cannot attach properties to text.
Solution:   First part of adding text properties.
diff --git a/src/proto.h b/src/proto.h
index 8570af9..92d9714 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -183,6 +183,9 @@
 # if defined(HAVE_TGETENT) && (defined(AMIGA) || defined(VMS))
 #  include "termlib.pro"
 # endif
+# ifdef FEAT_TEXT_PROP
+#  include "textprop.pro"
+# endif
 # include "ui.pro"
 # include "undo.pro"
 # include "userfunc.pro"