{input,decoder}/ffmpeg: move ffmpeg_domain to lib/ffmpeg/Domain.cxx

Eliminate duplicate definition (in input plugin and decoder plugin).

src/decoder/plugins/FfmpegDecoderPlugin.cxx

@@ -22,6 +22,7 @@
 
 #include "config.h"
 #include "FfmpegDecoderPlugin.hxx"
+#include "lib/ffmpeg/Domain.hxx"
 #include "../DecoderAPI.hxx"
 #include "FfmpegMetaData.hxx"
 #include "tag/TagHandler.hxx"
@@ -47,8 +48,6 @@ extern "C" {
 #include <assert.h>
 #include <string.h>
 
-static constexpr Domain ffmpeg_domain("ffmpeg");
-
 /* suppress the ffmpeg compatibility macro */
 #ifdef SampleFormat
 #undef SampleFormat