Fix build with gcc8

[thiblahute]

Fails with:

../subprojects/dssim/src/dssim.c: In function ‘dssim_preprocess_channel’:
../subprojects/dssim/src/dssim.c:629:34: error: passing argument 3 to restrict-qualified parameter aliases with argument 1 [-Werror=restrict]
        blur(chan->img, tmp, chan->img, width, height);
            ~~~~~~~~~       ~~~~^~~~~
../subprojects/dssim/src/dssim.c:639:38: error: passing argument 3 to restrict-qualified parameter aliases with argument 1 [-Werror=restrict]
    blur(chan->img_sq_blur, tmp, chan->img_sq_blur, width, height);
        ~~~~~~~~~~~~~~~~~       ~~~~^~~~~~~~~~~~~
../subprojects/dssim/src/dssim.c: In function ‘get_img1_img2_blur’:
../subprojects/dssim/src/dssim.c:657:5: error: passing argument 3 to restrict-qualified parameter aliases with argument 1 [-Werror=restrict]
    blur(img2, tmp, img2, width, height);
    ^~~~

[kornelski]

Thanks

[thiblahute]

Thanks you for merging ;) ooc, any chance you make new releases on the C branch?