veyron/runtimes/google/ipc: Set IsMounttable correctly
veyron/runtimes/google/ipc: Set IsMounttable correctly

Before this change, the IsMounttable bit wasn't set correctly on the
endpoint when listening via the proxy.

Change-Id: Ic83e3bcbc2e5ab1416c3148f63f54f5af0dd4cf8

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref