rhel: openvswitch-kmod-fedora.spec.in file bug fix

rhel: openvswitch-kmod-fedora.spec.in file bug fix

Patch 22c33c303932 used /usr/src/linux/<kernel version> as path of the
linux headers, which does not work for SLES. Use /lib/modules/<kernel
version>/build instead.

Fixes 22c33c303932 (rhel: support kmod build against mulitple kernel versions,
fedora)

Signed-off-by: Martin Xu <martinxu9.ovs@gmail.com>
CC: Greg Rose <gvrose8192@gmail.com>
CC: Markos Chandras <mchandras@suse.de>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Reviewed-by: Markos Chandras <mchandras@suse.de>