avcodec: use HAVE_THREADS header guards to silence -Wunused-function

When compiled with --disable-pthreads, e.g
http://fate.ffmpeg.org/report.cgi?time=20150917015044&slot=alpha-debian-qemu-gcc-4.7,
a bunch of -Wunused-functions are reported due to missing header guards
around threading related functions.
This patch should silence such warnings.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
