rtld: populate DT_DEBUG iff DYNAMIC segment is writable

rtld.h was accidentally missed in r293201

libexec/rtld-elf/rtld.h

@@ -264,6 +264,7 @@ typedef struct Struct_Obj_Entry {
     bool valid_hash_sysv : 1;	/* A valid System V hash hash tag is available */
     bool valid_hash_gnu : 1;	/* A valid GNU hash tag is available */
     bool dlopened : 1;		/* dlopen()-ed (vs. load statically) */
+    bool writable_dynamic : 1;	/* PT_DYNAMIC is writable */
 
     struct link_map linkmap;	/* For GDB and dlinfo() */
     Objlist dldags;		/* Object belongs to these dlopened DAGs (%) */