fix: lttng-probe-kvm-x86-mmu build with linux 6.6

fix: lttng-probe-kvm-x86-mmu build with linux 6.6

A small change was made upstream in `spte.h` that requires
`arch/x86/kvm` to be added to the search path when
building lttng-probe-kvm.x86-mmu.o.

See upstream commit :

  commit d10f3780bc2f80744d291e118c0c8bade54ed3b8
  Author: Sean Christopherson <seanjc@google.com>
  Date:   Tue Aug 8 15:40:59 2023 -0700

      KVM: x86/mmu: Include mmu.h in spte.h

      Explicitly include mmu.h in spte.h instead of relying on the "parent" to
      include mmu.h.  spte.h references a variety of macros and variables that
      are defined/declared in mmu.h, and so including spte.h before (or instead
      of) mmu.h will result in build errors, e.g.

Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I5c3fc87d3b006cefbcca198e6e15868a342cb8dd