x86/setup: Explicitly include acpi.h

After commit 342f43af

 ("iscsi_ibft: fix crash due to KASLR physical
memory remapping") x86_64_defconfig shows the following errors:

  arch/x86/kernel/setup.c: In function ‘setup_arch’:
  arch/x86/kernel/setup.c:916:13: error: implicit declaration of function ‘acpi_mps_check’ [-Werror=implicit-function-declaration]
    916 |         if (acpi_mps_check()) {
        |             ^~~~~~~~~~~~~~
  arch/x86/kernel/setup.c:1110:9: error: implicit declaration of function ‘acpi_table_upgrade’ [-Werror=implicit-function-declaration]
   1110 |         acpi_table_upgrade();
        |         ^~~~~~~~~~~~~~~~~~
  [... more acpi noise ...]

acpi.h was being implicitly included from iscsi_ibft.h in this
configuration so the removal of that header means these functions have
no definition or declaration.

In most other configurations, <linux/acpi.h> continued to be included
through at least <linux/tboot.h> if CONFIG_INTEL_TXT was enabled, and
there were probably other implicit include paths too.

Add acpi.h explicitly so there is no more error, and so that we don't
continue to depend on these unreliable implicit include paths.

Tested-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Cc: Maurizio Lombardi <mlombard@redhat.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Konrad Rzeszutek Wilk <konrad@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>