ANDROID: random: fix CRC issues with the merge

The upstream commit 9342656c ("random: remove unused tracepoints")
removed the random.h tracepoints which lead to CRC differences for the
following symbols:

    add_random_ready_callback()
    del_random_ready_callback()

If I add back #includes <linux/writeback.h> which is what was included
by the random.h tracepoint header, then the CRCs match the original CRC
values for those two functions above. This is necessary to retain GKI
compatibility.

Fixes: 9342656c ("random: remove unused tracepoints")
Change-Id: I52422404ec46273cc686d1930102e066cef05eb0
Signed-off-by: Will McVicker <willmcvicker@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>