patch 9.0.0144: text property cannot override 'cursorline' highlight

Problem:    Text property cannot override 'cursorline' highlight.
Solution:   Add the "override" flag to prop_type_add(). (closes #5533,
            closes #8225).
diff --git a/src/structs.h b/src/structs.h
index e6b9f0d..5b05c64 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -834,6 +834,7 @@
 #define PT_FLAG_INS_START_INCL	1	// insert at start included in property
 #define PT_FLAG_INS_END_INCL	2	// insert at end included in property
 #define PT_FLAG_COMBINE		4	// combine with syntax highlight
+#define PT_FLAG_OVERRIDE	8	// override any highlight
 
 // Sign group
 typedef struct signgroup_S