[S390] vdso: add missing vdso_install target

Add missing vdso_install target to install the unstripped vdso images
into $(MODLIB)/vdso/.  These files are helpful when containing
additional debugging information.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>