avfilter/vf_convolution: Fix compilation with sobel disabled

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/convolution.h

@@ -119,7 +119,7 @@ static void filter16_sobel(uint8_t *dstp, int width,
     }
 }
 
-static av_unused void ff_sobel_init(ConvolutionContext *s, int depth, int nb_planes)
+static inline void ff_sobel_init(ConvolutionContext *s, int depth, int nb_planes)
 {
     for (int i = 0; i < 4; i++) {
         s->filter[i] = filter_sobel;

libavfilter/vf_convolution.c

@@ -761,8 +761,10 @@ static int param_init(AVFilterContext *ctx)
             s->rdiv[i] = s->scale;
             s->bias[i] = s->delta;
         }
+#if CONFIG_SOBEL_FILTER
     } else if (!strcmp(ctx->filter->name, "sobel")) {
         ff_sobel_init(s, s->depth, s->nb_planes);
+#endif
     } else if (!strcmp(ctx->filter->name, "kirsch")) {
         for (i = 0; i < 4; i++) {
             s->filter[i] = filter_kirsch;