x86: fix vdso_install breaks user "make install"

I suggest to make the vdso_install step independent as
in following patch.

This solves the issue at ahnd and still gives us the posibility
to install the files should they be needed.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>