random: fxrng: Add an entry for RANDOM_RANDOMDEV to the source table

Otherwise we get a NULL pointer dereference when writing to /dev/random.

PR: 288826
Reviewed by: cem
MFC after: 1 week
Fixes: fa8db724ae6e ("random: Treat writes to /dev/random as separate from /entropy")
Differential Revision: https://reviews.freebsd.org/D52633

(cherry picked from commit 174d5d9397c492aff67f7ad49e130039697dc1dd)