staging:lustre:lustre:ptlrpc: fix sparse warnings in lproc_ptlrpc.c

Fixed the below warning in sparse:

drivers/staging/lustre/lustre/ptlrpc/lproc_ptlrpc.c:184:6:
warning: symbol 'ptlrpc_lprocfs_register' was not declared.
Should it be static?

Signed-off-by: Fred Chou <fred.chou.nd@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>