postprocess: fix black images with dcraw

for some reason on Debian dcraw creates black images, when -w (Use
camera white balance) isn't used.

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1101360

data/postprocess.sh

@@ -58,7 +58,7 @@ elif command -v "dcraw" > /dev/null
 then
 	DCRAW=dcraw
 	TIFF_EXT="tiff"
-	set --
+	set -- -w
 fi
 
 CONVERT=""