core: Fix asm-defines.h dependencies tracking

Previously, a change to headers like <asm/percpu.h> didn't trigger
a rebuild of asm-defines.s (thus asm-defines.h). This is fixed now.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>