Currently, build fails on FreeBSD with:

  CC       libvirt_driver_la-nodeinfo.lo
nodeinfo.c:1941:56: error: use of undeclared identifier 'SYSFS_SYSTEM_PATH'
    const char *prefix = sysfs_prefix ? sysfs_prefix : SYSFS_SYSTEM_PATH;
                                                       ^
1 error generated.

This is caused by commit b97b3048 that added sysfs_prefix to
nodeCapsInitNUMA and used SYSFS_CPU_PATH.

Fix it by unconditionally defining SYSFS_CPU_PATH instead of defining it
under #ifdef __linux__.