fate: Explicitly use gray16le in fate-sgi-gray16

This fixes the test on big endian.

Signed-off-by: Martin Storsjö <martin@martin.st>
Merge commit '66d04c068a30751750818dcfbb6555ab74eb3f6d'

* commit '66d04c068a30751750818dcfbb6555ab74eb3f6d':
  fate: Explicitly use gray16le in fate-sgi-gray16

Conflicts:
	tests/fate/image.mak

Merged-by: Michael Niedermayer <michaelni@gmx.at>