patch 8.1.1276: cannot combine text properties with syntax highlighting

Problem:    Cannot combine text properties with syntax highlighting.
Solution:   Add the "combine" field to prop_type_add(). (closes #4343)
diff --git a/src/structs.h b/src/structs.h
index c992e2a..fa8a765 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -727,6 +727,7 @@
 
 #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
 
 // Sign group
 typedef struct signgroup_S