mjpegtools: fix build

pkgs/tools/video/mjpegtools/default.nix

···
   buildInputs = [ libdv libjpeg libpng pkgconfig ]
               ++ lib.optional (!withMinimal) [ gtk libX11 SDL SDL_gfx ];
 
+  NIX_CFLAGS_COMPILE = [ "-I${SDL}/include/SDL" ];
+
   postPatch = ''
     sed -i -e '/ARCHFLAGS=/s:=.*:=:' configure
   '';