lib/oe/sstatesig: exclude ccache-native from signature hashes

lib/oe/sstatesig: exclude ccache-native from signature hashes

Enabling ccache should not cause a complete rebuild, so filter out ccache-native
from the dependencies the same way we do for quilt-native (so the world doesn't
repatch if quilt changes).

This doesn't effect the actual dependencies, just the dependencies that impact
the hash.

[ YOCTO #11417 ]

Signed-off-by: Ross Burton <ross.burton@intel.com>