random: Mark random_read as __maybe_unused

Fix unused function warning subsequent to f9ca1a2 ("random: always use /dev/urandom")

This silences the following compiler warning:

../drivers/char/random.c:1841:1: warning: unused function 'random_read' [-Wunused-function]
random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
^

Signed-off-by: Adam W. Willis <return.of.octobot@gmail.com>