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

This patch moves extern declaration from genops.c to obd_class.h,
addressing checkpatch.pl warning: externs should be avoided in .c

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