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/feature.h b/src/feature.h
index 9e87345..9615a9f 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -502,6 +502,13 @@
 #endif
 
 /*
+ * +textprop		Text properties
+ */
+#if defined(FEAT_EVAL) && defined(FEAT_SYN_HL)
+# define FEAT_TEXT_PROP
+#endif
+
+/*
  * +spell		spell checking
  *
  * Disabled for EBCDIC: * Doesn't work (SIGSEGV).