lavfi/nnedi: Fix a compilation warning.

Silences the following warning:
libavfilter/vf_nnedi.c:611:15: warning: assignment discards ‘const’ qualifier from pointer target type

libavfilter/vf_nnedi.c

@@ -601,7 +601,7 @@ static void evalfunc_1(NNEDIContext *s, FrameData *frame_data)
 
         const int ystart = frame_data->field[plane];
         const int ystop = height - 12;
-        uint8_t *srcpp;
+        const uint8_t *srcpp;
 
         if (!(s->process_plane & (1 << plane)))
             continue;