icy_server: don't export icy_server_metadata_string()

icy_server_metadata_string() is only called by
icy_server_metadata_page().

src/icy_server.c

@@ -53,7 +53,7 @@ icy_server_metadata_header(const char *name,
 			       content_type);
 }
 
-char*
+static char *
 icy_server_metadata_string(const char *stream_title, const char* stream_url)
 {
 	gchar *icy_metadata;