solderparty_rp2350_stamp_xl: Properly mark as using RP2350B

[arturo182]

I saw the define was needed for the PGA2350, so I think it needs to be here as well.

[earlephilhower]

D'oh! I actually use the XL as the libpico SDK board since it forces the inclusion of the -B logic (which is commented out on the -A).

Thx!

[arturo182]

Yes, I noticed that and then went to double check if it was actually declaring itself as a -B, but it is now :)