diffstat: add nativesdk to BBCLASSEXTEND

The diffstat tool is part of HOSTTOOLS. To support hosts that do not
have it installed with buildtools-tarball it must be enabled for
nativesdk.

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>