ast: Rename ast.c -> astdec.c

Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/Makefile

@@ -51,7 +51,7 @@ OBJS-$(CONFIG_ASF_DEMUXER)               += asfdec.o asf.o asfcrypt.o \
 OBJS-$(CONFIG_ASF_MUXER)                 += asfenc.o asf.o
 OBJS-$(CONFIG_ASS_DEMUXER)               += assdec.o
 OBJS-$(CONFIG_ASS_MUXER)                 += assenc.o
-OBJS-$(CONFIG_AST_DEMUXER)               += ast.o
+OBJS-$(CONFIG_AST_DEMUXER)               += astdec.o
 OBJS-$(CONFIG_AU_DEMUXER)                += au.o pcm.o
 OBJS-$(CONFIG_AU_MUXER)                  += au.o
 OBJS-$(CONFIG_AVI_DEMUXER)               += avidec.o