Silence uninteresting warnings.

Upstream doesn't seem to care about signed/unsigned or pointer arithmetic
warnings, so there's little point us looking at them.

The remaining one is weird though --- given that this is an inline function
in os/os-linux.h, how does this link?

  external/fio/idletime.c: In function 'idle_prof_thread_fn':
  external/fio/idletime.c:92:2: warning: implicit declaration of function 'fio_set_sched_idle' [-Wimplicit-function-declaration]
    if ((retval = fio_set_sched_idle()))
    ^

Change-Id: I27efbe3b18a46a5253c2edd464b655d995c82024