netfilter: xtables: Bring SPDX identifier back

netfilter: xtables: Bring SPDX identifier back

[ Upstream commit 20646f5b1e798bcc20044ae90ac3702f177bf254 ]

Commit e2be04c7f995 ("License cleanup: add SPDX license identifier to
uapi header files with a license") added the correct SPDX identifier to
include/uapi/linux/netfilter/xt_IDLETIMER.h.

A subsequent commit removed it for no reason and reintroduced the UAPI
license incorrectness as the file is now missing the UAPI exception
again.

Add it back and remove the GPLv2 boilerplate while at it.

Fixes: 68983a354a65 ("netfilter: xtables: Add snapshot of hardidletimer target")
Cc: Manoj Basapathi <manojbm@codeaurora.org>
Cc: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: netfilter-devel@vger.kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>