staging: lustre: fix non-static symbol warnings reported by sparse

Warnings reported by sparse:

drivers/staging/lustre/lustre/ptlrpc/pinger.c:94:5:
warning: symbol 'ptlrpc_ping' was not declared. Should it be static?

drivers/staging/lustre/lustre/ptlrpc/pinger.c:113:6:
warning: symbol 'ptlrpc_update_next_ping' was not declared. Should it be static

drivers/staging/lustre/lustre/ptlrpc/pinger.c:144:6:
warning: symbol 'pinger_check_timeout' was not declared. Should it be static?

drivers/staging/lustre/lustre/ptlrpc/pinger.c:425:21:
warning: symbol 'ptlrpc_new_timeout' was not declared. Should it be static?

drivers/staging/lustre/lustre/ptlrpc/pinger.c:551:1:
warning: symbol 'pet_list' was not declared. Should it be static?

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>