netfilter: xtables: Bring SPDX identifier back

netfilter: xtables: Bring SPDX identifier back

[ Upstream commit 6947c0102dce2591a59f8957c8c22f88fc255360 ]

Commit cdb51be7ebd5 ("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: c4e9d501ee48 ("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>