patch 8.1.0582: text properties are not enabled

Problem:    Text properties are not enabled.
Solution:   Fix sizeof argument and re-enable the text properties feature.
            Fix memory leak.
diff --git a/src/feature.h b/src/feature.h
index 404461c..9615a9f 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -505,7 +505,7 @@
  * +textprop		Text properties
  */
 #if defined(FEAT_EVAL) && defined(FEAT_SYN_HL)
-// # define FEAT_TEXT_PROP
+# define FEAT_TEXT_PROP
 #endif
 
 /*