mk: fix combined library build with Xen driver

mk: fix combined library build with Xen driver

Building RTE_LIBRTE_PMD_XENVIRT was broken when RTE_BUILD_COMBINE_LIBS was
enabled (http://dpdk.org/ml/archives/dev/2015-November/028660.html).
Now the underlying issue is rather simple, the xen code needs libxenstore.
But rte.app.mk so far only considered that when RTE_BUILD_COMBINE_LIBS was
disabled.
While it is correct to create the DPDK sublib linking only in the
RTE_BUILD_COMBINE_LIBS=n case, the libxenstore should be added to the linked
libs in any case if RTE_LIBRTE_PMD_XENVIRT is enabled.

Reported-by: Thiago Martins <thiagocmartinsc@gmail.com>
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>