kbuild: verify asm-generic header list
In order to integrate the system call header generation with generating
the asm-generic wrappers, restrict the generated headers to those that
actually exist in include/asm-generic/.
The path is already known, so add these as a dependency.
The asm-generic/bugs.h header was removed in commit 61235b24b9cb ("init:
Remove check_bugs() leftovers"), which now causes a build failure, so
drop it from the list.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>