Add GCP fetcher to list of supported protocols

If accepted, this patch should merge with the corresponding BitBake
patch titled "fetch2: add Google Cloud Platform (GCP) fetcher".

Signed-off-by: Emil Ekmečić <eekmecic@snap.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>