decoder: make all decoder_plugin structs const

All decoder_plugin structs are initialized at compile time, and must
never change.

src/decoder/mpc_plugin.c

@@ -297,7 +297,7 @@ static struct tag *mpcTagDup(const char *file)
 
 static const char *mpcSuffixes[] = { "mpc", NULL };
 
-struct decoder_plugin mpcPlugin = {
+const struct decoder_plugin mpcPlugin = {
 	.name = "mpc",
 	.stream_decode = mpc_decode,
 	.tag_dup = mpcTagDup,