drivers/char/random.c: make primary_crng static

Since the definition of struct crng_state is private to random.c, and
primary_crng is neither declared or used elsewhere, there's no reason
for that symbol to have external linkage.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>