avcodec/x86/rv40dsp_init: silence -Wunused-variable on --disable-mmx

This silences -Wunused-variable when compiled with --disable-mmx, e.g
http://fate.ffmpeg.org/log.cgi?time=20150919094617&log=compile&slot=x86_64-archlinux-gcc-disable-mmx.
The alternative of header guards will make it far too ugly.

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