x86: apic.c: xapic_icr_read and x2apic_icr_read should be static

Impact: cleanup, reduce kernel size a bit, avoid sparse warning

Fixes sparse warning:
arch/x86/kernel/apic.c:270:5: warning: symbol 'x2apic_icr_read' was not declared. Should it be static?

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>