lustre: ptlrpc: add "ptlrpc_internal.h" to sec_gc.c

ptlrpc_internal.h contains the prototypes for sptlrpc_gc_init() and
sptlrpc_gc_fini(), which are defined in sec_gc.c.

This removes the following sparse warnings:

drivers/staging/lustre/lustre/ptlrpc/sec_gc.c:217:5: warning: symbol 'sptlrpc_gc_init' was not declared. Should it be static?
drivers/staging/lustre/lustre/ptlrpc/sec_gc.c:241:6: warning: symbol 'sptlrpc_gc_fini' was not declared. Should it be static?

Signed-off-by: Spencer Baugh <sbaugh@andrew.cmu.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>