cmdutils: silence unused warnings under --disable-swscale, --disable-swresample

This patch silences such warnings by placing initializations under a header guard,
see e.g
http://fate.ffmpeg.org/log.cgi?time=20150919095430&log=compile&slot=x86_64-archlinux-gcc-disableswscale,
http://fate.ffmpeg.org/log.cgi?time=20150919095048&log=compile&slot=x86_64-archlinux-gcc-disableswresample
for examples of such warnings.
It also has the benefit of placing library specific #defines next to each other.

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