lavfi: rename sink_buffer.c into buffersink.c.

libavfilter/Makefile

@@ -33,13 +33,13 @@ OBJS = allfilters.o                                                     \
        avfilter.o                                                       \
        avfiltergraph.o                                                  \
        buffer.o                                                         \
+       buffersink.o                                                     \
        buffersrc.o                                                      \
        drawutils.o                                                      \
        fifo.o                                                           \
        formats.o                                                        \
        graphdump.o                                                      \
        graphparser.o                                                    \
-       sink_buffer.o                                                    \
        transform.o                                                      \
        video.o                                                          \