swscale/swscale_unscaled: fix gbrap10be md5 different on big endian system

swscale/swscale_unscaled: fix gbrap10be md5 different on big endian system

You can reproduce it by below command:
./ffmpeg -f lavfi -i "testsrc=duration=1:rate=30" -vf format=gbrap10 -vcodec rawvideo \
    -pix_fmt gbrap10le -flags +bitexact -sws_flags +accurate_rnd+bitexact -fflags +bitexact  \
    -frames:v 1 -f nut md5:

little-endian:
f91e2edd8098276579c1929e5e160416
big-endian:
ba4d011dbbdc78ccbf6cc7d698630929

Signed-off-by: Limin Wang <lance.lmwang@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>