When building with W=1 after commit 80b6093b ("kbuild: add -Wundef
to KBUILD_CPPFLAGS for W=1 builds"), the following warning occurs.

  In file included from arch/powerpc/kvm/bookehv_interrupts.S:26:
  arch/powerpc/kvm/../kernel/head_booke.h:20:6: warning: "THREAD_SHIFT" is not defined, evaluates to 0 [-Wundef]
     20 | #if (THREAD_SHIFT < 15)
        |      ^~~~~~~~~~~~

THREAD_SHIFT is defined in thread_info.h but it is not directly included
in head_booke.h, so it is possible for THREAD_SHIFT to be undefined. Add
the include to ensure that THREAD_SHIFT is always defined.
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/202304050954.yskLdczH-lkp@intel.com/Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230406-wundef-thread_shift_booke-v1-1-8deffa4d84f9@kernel.org