swscale: fix compilation if avcodec and avformat are disabled
swscale: fix compilation if avcodec and avformat are disabled

(cherry picked from commit 0b9f0d211699248112818b50e5b68d22c996cf3f)

jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2