use ssize_t

The corresponding varnish API function arguments are signed, make our
struct match them.

src/vfp_brotli.c

@@ -70,8 +70,8 @@ struct vbr {
 		BrotliDecoderState *dec;
 	}			state;
 	uint8_t			*buf;
-	size_t			bufsz;
-	size_t			buflen;
+	ssize_t			bufsz;
+	ssize_t			buflen;
 	enum vbr_which		which;
 };