gdb: use the location_completer for the list command

The "location_completer" function has been available for a long time,
but it was seemingly never used as the completer for the list function.
A quick check through git history shows that a similar completer was
available for the "edit" command but wasn't added to "list" with no
reasoning for it.

I think "list" should use the location_completer, as it is more aware of
the locspec grammar.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/cli/cli-cmds.c

@@ -2898,6 +2898,7 @@ This can be changed using \"set listsize\", and the current value\n\
 can be shown using \"show listsize\"."));
 
   add_com_alias ("l", list_cmd, class_files, 1);
+  set_cmd_completer(list_cmd, location_completer);
 
   c = add_com ("disassemble", class_vars, disassemble_command, _("\
 Disassemble a specified section of memory.\n\