Manually fix a pidl bug - that field should be an FT_STRING.

No, I don't know why it's making it FT_NONE; it's a bit of a weird data
type, with a string inside a structure.

Change-Id: I27a6d7577ef4a9f4da8ddad2cad97ad097135e90
Reviewed-on: https://code.wireshark.org/review/5685
Reviewed-by: Guy Harris <guy@alum.mit.edu>