Add Kbuild file for Alpha 'make headers_install'

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

include/asm-alpha/Kbuild

+include include/asm-generic/Kbuild.asm
+
+unifdef-y += console.h fpu.h sysinfo.h
+
+header-y += gentrap.h regdef.h pal.h reg.h