staging: lustre: remove libcfs_debug_set_level prototype from libcfs_private.h

The function libcfs_debug_set_level is used only internally so no reason
to expose it in libcfs_private.h. This is broken out from LU-5829 patch
http://review.whamcloud.com/13319

.

Signed-off-by: frank zago <fzago@cray.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>