libswscale/swscale: fix -Wunused-function

hyscale, hcscale are only used in old filter code, hence place
header guard to silence -Wunused-function.

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