bitbake.conf: Add gsutil as hosttool for gcp fetcher.

bitbake.conf: Add gsutil as hosttool for gcp fetcher.

The gcp fetcher uses gsutil to check if the file exists before
fetching. This change ensures the tool is included as a nonfatal
hosttool so that it's included in the build environment when
available.

Signed-off-by: Charlie Johnston <charlie.johnston@loftorbital.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>