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 â��cpu_dr7â�=99
      9 | DECLARE_PER_CPU(unsigned long, cpu_dr7);
        |                                ^~~~~~~

Signed-off-by: Sean Christopherson <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

Author: sean-jc

arch/x86/include/asm/debugreg.h

@@ -2,8 +2,8 @@
 #ifndef _ASM_X86_DEBUGREG_H
 #define _ASM_X86_DEBUGREG_H
 
-
 #include <linux/bug.h>
+#include <linux/percpu.h>
 #include <uapi/asm/debugreg.h>
 
 DECLARE_PER_CPU(unsigned long, cpu_dr7);