x86/debug: Include percpu.h in debugreg.h to get DECLARE_PER_CPU() et al

Include percpu.h to pick up the definition of DECLARE_PER_CPU() and
friends instead of relying on the parent to provide the #include.  E.g.
swapping the order of includes in arch/x86/kvm/vmx/nested.c (simulating
KVM code movement being done for other purposes) results in build errors:

  In file included from arch/x86/kvm/vmx/nested.c:3:
  arch/x86/include/asm/debugreg.h:9:32: error: unknown type name ‘cpu_dr7â€=99
      9 | DECLARE_PER_CPU(unsigned long, cpu_dr7);
        |                                ^~~~~~~

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221110201707.1976032-1-seanjc@google.com