multilib_header: avoid sstate checksum issues for -nativesdk recipes

multilib_header: avoid sstate checksum issues for -nativesdk recipes

Much as with -native recipes, as addressed in commit
b15730caf0d4c40271796887505507f2501958bb, arch specific variables
like MIPSPKGSFX_ABI were affecting -nativesdk sstate checksums for
recipes like nativesdk-glibc-initial.

Disable multilib_header for nativesdk as we don't use multilibs in
this scenario.

[YOCTO #10320]

(From OE-Core rev: f1c7b4f16dc9a7e5155108641fed8b3d98c931f3)

Signed-off-by: Joshua Lock <joshua.g.lock@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>