staging/lustre: clean function declaration in cl_io.c up

We get 1 warning when building kernel with W=1:
drivers/staging/lustre/lustre/obdclass/cl_page.c:488:6: warning: no previous prototype for 'cl_page_disown0' [-Wmissing-prototypes]

In fact, this function is declared in
drivers/staging/lustre/lustre/obdclass/cl_io.c, but should be
removed in a header file. thus can be recognized in other file.

So this patch moves the declaration into
drivers/staging/lustre/lustre/include/cl_object.h.

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