staging/lustre/libcfs_string.h: don't export internal functions

cfs_expr_list_print is removed due to no callers. Other internal
functions are held unexported.

Cc: Andreas Dilger <andreas.dilger@intel.com>
Cc: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>