staging: lustre: ptlrpc: add missing include directive

Without including ptlrpc_internal.h, GCC gives prototype warnings
"pack_generic.c:642:5: warning: no previous prototype for ..."
and sparse also complains "pack_generic.c:642:5: warning: symbol
'lustre_unpack_req_ptlrpc_body' was not declared. ..."

Signed-off-by: Ioan-Adrian Ratiu <adi@adirat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>