lavfi/negate: unbreak negate alpha

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/vf_lut.c

@@ -403,7 +403,7 @@ DEFINE_LUT_FILTER(lutrgb, "Compute and apply a lookup table to the RGB input vid
 #if CONFIG_NEGATE_FILTER
 
 static const AVOption negate_options[] = {
-    { "negate_alpha", NULL, OFFSET(negate_alpha), AV_OPT_TYPE_INT, { .i64 = 0 }, .flags = FLAGS },
+    { "negate_alpha", NULL, OFFSET(negate_alpha), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, 1, FLAGS },
     { NULL },
 };