Needed for Release 2.2 !

Does not apply to 5.0 which has no linker_set declared or used, just
	#include <sys/linker_set.h>

*** 4.8-RELEASE/src/sys/ddb/db_command.c	Mon Jul 30 00:48:36 2001
--- new/src/sys/ddb/db_command.c	Wed Apr  2 19:06:25 2003
***************
*** 57,63 ****
  db_addr_t	db_last_addr;
  db_addr_t	db_prev;
  db_addr_t	db_next;
! extern struct linker_set	db_show_cmd_set;
  
  static db_cmdfcn_t	db_fncall;
  static db_cmdfcn_t	db_gdb;
--- 60,66 ----
  db_addr_t	db_last_addr;
  db_addr_t	db_prev;
  db_addr_t	db_next;
! struct linker_set	db_show_cmd_set;
  
  static db_cmdfcn_t	db_fncall;
  static db_cmdfcn_t	db_gdb;