more exclusion of SkImageEncoder_none.cpp

The current roll is failing due to double linking:

    multiple definition of 'SkEncodeImage(SkWStream*, SkPixmap const&, SkEncodedImageFormat, int)'
    src/images/SkImageEncoder.pic.o: previous definition here

Change-Id: I64ada3591b4c4c0cb8deada0dc796d9a0376b6a7
Reviewed-on: https://skia-review.googlesource.com/18855
Reviewed-by: Matt Sarett <msarett@google.com>
1 file changed