x86: fix sparse warnings in acpi/bus.c

Add function definition and extern variables to asm-x86/acpi.h.

All of these are used in bus.c in ifdef(CONFIG_X86) sections, so are
only added to the x86 include headers.  boot.c already includes acpi.h
so no changes are needed there.

Fixes the following:
arch/x86/kernel/acpi/boot.c:83:4: warning: symbol 'acpi_sci_flags' was not declared. Should it be static?
arch/x86/kernel/acpi/boot.c:84:5: warning: symbol 'acpi_sci_override_gsi' was not declared. Should it be static?
arch/x86/kernel/acpi/boot.c:421:13: warning: symbol 'acpi_pic_sci_set_trigger' was not declared. Should it be static?

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>