Fix a compiler warning: unused GetDevURandom()

Summary:

random.cpp:255:13: error: unused function 'GetDevURandom' [-Werror,-Wunused-function]

Clang 9.0.0, FreeBSD 12.1

Silence by planting a dummy reference to the GetDevURandom symbol
in the places where we don't call the function.

We see the same warnings in our CI logs.

This is a backport of core#17563

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10090