kbuild: export linux/{a.out,kvm,kvm_para}.h on headers_install_all

Export linux/a.out.h, linux/kvm.h and linux/kvm_para.h on
headers_install_all if at least one architecture has appropriate files
in arch-dependent headers.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Michal Marek <mmarek@suse.cz>