lavfi: change AVFilterBufferRefAudioProps.sample_rate from uint32_t to int

There's no reason for it to be explicitly 32 bits. It's declared as a
plain int in all other places in Libav.

This breaks audio filtering API and ABI in theory, but since it's
unusable right now this shouldn't be a problem.

libavfilter/avfilter.h

@@ -109,7 +109,7 @@ typedef struct AVFilterBuffer {
 typedef struct AVFilterBufferRefAudioProps {
     uint64_t channel_layout;    ///< channel layout of audio buffer
     int nb_samples;             ///< number of audio samples
-    uint32_t sample_rate;       ///< audio buffer sample rate
+    int sample_rate;            ///< audio buffer sample rate
     int planar;                 ///< audio buffer - planar or packed
 } AVFilterBufferRefAudioProps;