imgdiff: skip spurious gzip headers in image files

dragon kernel is compressed via lz4 for boot speed and bootloader
support reasons and recent prebuilts happen to include the gzip header
sequence which is causing imgdiff to fail.

Detect a spurious gzip header and treat the section as a normal section.

Bug: 26133184
Change-Id: I369d7d576fd7d2c579c0780fc5c669a5b6ea0d3d
(cherry picked from commit 0f2f6a746af517afca9e5e089a4a17be0a9766d6)
Signed-off-by: David Riley <davidriley@google.com>