decoder: make the suffixes and mime_types arrays really const

The strings were constant, but the pointers weren't.  C syntax is
somewhat tricky..

src/decoder/ffmpeg_plugin.c

@@ -359,14 +359,14 @@ static struct tag *ffmpeg_tag(const char *file)
  * only that files
  */
 
-static const char *ffmpeg_Suffixes[] = {
+static const char *const ffmpeg_Suffixes[] = {
 	"wma", "asf", "wmv", "mpeg", "mpg", "avi", "vob", "mov", "qt", "swf", "rm", "swf",
 	"mp1", "mp2", "mp3", "mp4", "m4a", "flac", "ogg", "wav", "au", "aiff", "aif", "ac3", "aac", "mpc",
 	NULL
 };
 
 //not sure if this is correct...
-static const char *ffmpeg_Mimetypes[] = {
+static const char *const ffmpeg_Mimetypes[] = {
 	"video/x-ms-asf",
 	"audio/x-ms-wma",
 	"audio/x-ms-wax",