Cygwin: posix timers: allocate timer_tracker on system heap.

Cygwin: posix timers: allocate timer_tracker on system heap.

Allocating on the cygheap would copy information of the tracker into
the child process.  A forked child knows the timer id and could simply
still access the (free'd but still valid) timer_tracker on the heap,
which is dangerous and very certainly doesn't reflect POSIX semantics.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>