truss: Add missing underscore to compat_prefix for FreeBSD32

I accidentally dropped this in the final version of D27625, so it didn't
actually work as intended. I found this while testing the MFC to stable/13.

MFC after: immediately
Fixes: 7daca4e2043f ("truss: improved support for decoding compat32 arguments")

(cherry picked from commit 3cbad8287aa0ed72e07df1130ce2ae490642e63a)