configure: fix build issue of vf_dnn_processing.c when --disable-swscale

vf_dnn_processing.c recently changed to use swscale to trasfer data
between AVFrame and dnn model.
Signed-off-by: Guo, Yejun <yejun.guo@intel.com>
Signed-off-by: Linjie Fu <linjie.fu@intel.com>

configure

@@ -3498,6 +3498,7 @@ derain_filter_select="dnn"
 deshake_filter_select="pixelutils"
 deshake_opencl_filter_deps="opencl"
 dilation_opencl_filter_deps="opencl"
+dnn_processing_filter_deps="swscale"
 dnn_processing_filter_select="dnn"
 drawtext_filter_deps="libfreetype"
 drawtext_filter_suggest="libfontconfig libfribidi"