[IA64] build arch/ia64/kernel/acpi-ext.o when CONFIG_ACPI

Simplify the makefile slightly by always building acpi-ext.o when
CONFIG_ACPI is turned on.

Yes, this adds a little bloat to the other configs, but not much:
   text	   data	    bss	    dec	    hex	filename
    839	     41	      0	    880	    370	arch/ia64/kernel/acpi-ext.o

Before:
   text	   data	    bss	    dec		    hex	filename
10952753	1299212	1334241	13586206	 cf4f1e	vmlinux

After:
   text	   data	    bss	    dec		    hex	filename
10953739	1299084	1334241	13587064	 cf5278	vmlinux

(gdb) p 13587064 - 13586206
$2 = 858

Seems like a small price to pay for the benefit of not having to think
so hard about the multitude of ia64 configs when reading code/Makefiles.

Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>