x86/cpu: Use EXPORT_PER_CPU_SYMBOL_GPL() for x86_spec_ctrl_current

Sparse rightfully complains:

  bugs.c:71:9: sparse: warning: incorrect type in initializer (different address spaces)
  bugs.c:71:9: sparse:    expected void const [noderef] __percpu *__vpp_verify
  bugs.c:71:9: sparse:    got unsigned long long *

The reason is that x86_spec_ctrl_current which is a per CPU variable is
exported with EXPORT_SYMBOL_GPL().

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/20240304005104.732288812@linutronix.de