ACPI: Fix build errors due objects compiled unconditionally

ACPI: Fix build errors due objects compiled unconditionally

If the CONFIG_ACPI Kconfig symbol is not enabled and a partial build is
attempted, compile errors will happen due missing types and identifiers.

This can be easily reproduced with the following commands:

$ export CROSS_COMPILE="arm-linux-gnueabihf-" ARCH=arm
$ make allmodconfig
$ make M=drivers/acpi/
  CC      drivers/acpi//tables.o
drivers/acpi//tables.c:235:3: warning: 'struct acpi_subtable_proc' declared inside parameter list
   unsigned int max_entries)
   ^
drivers/acpi//tables.c:235:3: warning: its scope is only this definition or declaration, which is probably not what you want
drivers/acpi//tables.c: In function 'acpi_parse_entries_array':
drivers/acpi//tables.c:269:4: error: invalid use of undefined type 'struct acpi_subtable_proc'
...
scripts/Makefile.build:258: recipe for target 'drivers/acpi//tables.o' failed
make[1]: *** [drivers/acpi//tables.o] Error 1
Makefile:1401: recipe for target '_module_drivers/acpi/' failed
make: *** [_module_drivers/acpi/] Error 2

This is because objects are tried to be built unconditionally even when
CONFIG_ACPI is not enabled. This is usually not a problem since arches'
Kconfig sources drivers/acpi/Kconfig directly and also selects ACPI but
the Makefile should conditionally build the objects as well to prevent
these build errors.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>