avcodec/x86/hpeldsp_rnd_template: silence -Wunused-function on --disable-mmx

This silences some of the -Wunused-function warnings when compiled with --disable-mmx, e.g
http://fate.ffmpeg.org/log.cgi?time=20150919094617&log=compile&slot=x86_64-archlinux-gcc-disable-mmx

.
Header guards are too brittle and ugly for this case.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>