format: Add fields in format descriptor for fixed headers

Very simple header support, but that's all we need for AMR

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

include/gapk/formats.h

@@ -53,6 +53,9 @@ struct format_desc {
 	unsigned int		frame_len;
 	fmt_conv_cb_t		conv_from_canon;
 	fmt_conv_cb_t		conv_to_canon;
+
+	unsigned int		header_len;
+	const uint8_t *		header;
 };
 
 const struct format_desc *fmt_get_from_type(enum format_type type);