Staging: lustre: Fix externs should be avoided in .c

Thi patch moves extern declaration to "obd_class.h".
Remove prototype from "class_obd.c".
Issue addressed by checkpatch.pl.

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>