x86/callthunks: Use EXPORT_PER_CPU_SYMBOL_GPL() for per CPU variables

Sparse complains rightfully about the usage of EXPORT_SYMBOL_GPL() for per
CPU variables:

  callthunks.c:346:20: sparse: warning: incorrect type in initializer (different address spaces)
  callthunks.c:346:20: sparse:    expected void const [noderef] __percpu *__vpp_verify
  callthunks.c:346:20: sparse:    got unsigned long long *

Use EXPORT_PER_CPU_SYMBOL_GPL() instead.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240304005104.841915535@linutronix.de