gbl: libgbl: src: decompress.rs: fix magic number of gzip

The magic number is not correct due to a refactoring. Fix this
to let gzipped kernel to work.

We also add a simple unit test for the gzip case.

Change-Id: I5730624a16d0239b312afb70a323e9df79ac6423
Signed-off-by: Ying-Chun Liu <liupa@google.com>