[S390] correct vdso version string

The glibc vdso code for s390 uses the version string 2.6.29, the
kernel uses the version string 2.6.26. No wonder the vdso code
is never used. The first kernel version to contain the vdso code
is 2.6.29 which makes this the correct version.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>