patch 8.1.2371: FEAT_TEXT_PROP is a confusing name

Problem:    FEAT_TEXT_PROP is a confusing name.
Solution:   Use FEAT_PROP_POPUP. (Naruhiko Nishino, closes #5291)
diff --git a/src/optionstr.c b/src/optionstr.c
index b63a351..98e90a4 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -2220,7 +2220,7 @@
     }
 #endif
 
-#ifdef FEAT_TEXT_PROP
+#ifdef FEAT_PROP_POPUP
     // 'previewpopup'
     else if (varp == &p_pvp)
     {