kheaders: rebuild kheaders_data.tar.xz when KBUILD_BUILD_TIMESTAMP is changed

kheaders: rebuild kheaders_data.tar.xz when KBUILD_BUILD_TIMESTAMP is changed

This problem is similar to commit 7f8256ae0efb ("initramfs: Encode
dependency on KBUILD_BUILD_TIMESTAMP"): kernel/gen_kheaders.sh has an
internal dependency on KBUILD_BUILD_TIMESTAMP that is not exposed to
make, so changing KBUILD_BUILD_TIMESTAMP will not trigger a rebuild
of the archive.

Move $(KBUILD_BUILD_TIMESTAMP) to the Makefile so that is is recorded
in the *.cmd file.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>