staging: lustre: obdclass: clean function declarations in obd_config.c and class_obd.c up

We get 3 warnings when building kernel with W=1:

drivers/staging/lustre/lustre/obdclass/obd_mount.c:705:5: warning: no previous prototype for 'lustre_check_exclusion' [-Wmissing-prototypes]
drivers/staging/lustre/lustre/obdclass/obd_mount.c:1226:5: warning: no previous prototype for 'lustre_register_fs' [-Wmissing-prototypes]
drivers/staging/lustre/lustre/obdclass/obd_mount.c:1231:5: warning: no previous prototype for 'lustre_unregister_fs' [-Wmissing-prototypes]

In fact, the first function is declared in
drivers/staging/lustre/lustre/obdclass/obd_config.c,
and the other two functions are declared in
drivers/staging/lustre/lustre/obdclass/class_obd.c,
but all of these functions should be declared in
a header file, thus can be recognized in other file.

So this patch moves the declarations into
drivers/staging/lustre/lustre/include/obd_class.h.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>