lib/oe/sstatesig.py: Exclude git-native from sstate checksum

lib/oe/sstatesig.py: Exclude git-native from sstate checksum

gcc exposed this issue where cross gcc recipes were not having
same task checksums as libgcc or gcc-runtime the target recipes
which use same shared workdir and it was triggering the unpack
fetch and patch tasks to reexecute and hence the trouble

Now that we have more than 1 package to consider lets combine
the check

Thanks RP for help and this is on the line of patch
793ce6cd9aa632e0f13789c8293770a86085d28d

(From OE-Core rev: 72cc3df204c4b838cc0358a7ac989adfdd5c1760)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>