xen.spec: Fix xenserver build failure.

This commit conducts similar operations as commit 2fd84d5e5
(rhel: Include pkgconfig and update header install in
Fedora/RHEL spec.) to xenserver spec file to prevent the
following rpmbuild failure.

  RPM build errors:
       Installed (but unpackaged) file(s) found:
           /usr/include/openflow/nicira-ext.h
           /usr/include/openflow/openflow-1.0.h
           /usr/include/openflow/openflow-1.1.h
           /usr/include/openflow/openflow-1.2.h
           /usr/include/openflow/openflow-1.3.h
           /usr/include/openflow/openflow-1.4.h
           /usr/include/openflow/openflow-1.5.h
           /usr/include/openflow/openflow-common.h
           /usr/include/openflow/openflow.h
           /usr/include/openvswitch/types.h
           /usr/lib/pkgconfig/libofproto.pc
           /usr/lib/pkgconfig/libopenvswitch.pc
           /usr/lib/pkgconfig/libovsdb.pc
           /usr/lib/pkgconfig/libsflow.pc

VMware-BZ: #1362092

Reported-by: Edwin Chiu <echiu@nicira.com>
Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>