mk: fix examples install
mk: fix examples install

Depending on non-doc targets being built before and the setting of DESTDIR
the copy of the examples dir being part of install-doc could in some cases
fail with a non existent "$(DESTDIR)$(datadir)" target directory.
Add the conditional rte_mkdir for that to avoid the issue.

Signed-off-by: Christian Ehrhardt <48da28e2c2cd455d60e19b14c46925e20a3c5d5b@canonical.com>
Acked-by: Thomas Monjalon <ee2fa8b2681c737f775687e475dcbf2028a412f1@6wind.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk