lavfi/signalstats: Cast the return value of AV_RN16() to int.

lavfi/signalstats: Cast the return value of AV_RN16() to int.

It is unsigned at least on arm, fixes the following warnings:
libavfilter/vf_signalstats.c:833:21: warning: taking the absolute value of unsigned type 'unsigned int' has no effect
            dify += abs(yuv - AV_RN16(prev->data[0] + pw + i * 2));