Adding some initializations

[barrelfish] / if / mt_waitset.if

interface mt_waitset "Multithreaded waitset test" {
    rpc rpc_method(in uint64 i1, in uint8 s[ss, 2048], in uint32 i2, out uint64 o1, out uint8 r[rs, 2048], out uint32 o2);
};