Remove mutex lock around sys_gettimeofday() in os:timestamp/0

[Tuncer Ayaz]

Remove mutex lock around sys_gettimeofday() in os:timestamp/0

In the erlang-questions thread "os:timestamp() uses mutex lock"
Jonas Falkevik questioned the need for a mutex lock in os:timestamp/0.
Kenneth Lundin confirmed that the lock is not needed and asked for
a patch.

git fetch git://github.com/tuncer/otp.git os_timestamp_no_mutex