subsys/random: fix unused value warning

When using LLVM/Clang, it complains about memcpy() being
casted to (void *):

  warning: expression result unused; should this cast be to 'void'? [-Wunused-value]

So change those to (void) instead as the return of memcpy()
is not used anyway.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>