tutorial: Fix type mismatch in example FFI code

tutorial: Fix type mismatch in example FFI code

Compile error:
time.rs:13:23: 13:43 error: mismatched types: expected
*R[tv_sec=mMltv_usec=mMl] but found *R[tv_sec=Mltv_usec=Ml] (record
elements differ in mutability)
time.rs:13     libc::gettimeofday(std::ptr::addr_of(x),
std::ptr::null());
                                  ^~~~~~~~~~~~~~~~~~~~
error: aborting due to previous errors
rust: upcall fail 'explicit failure', ../src/comp/driver/session.rs:70
rust: domain main @0x9dfd178 root task failed