posix: fix one-time timer for SIGEV_SIGNAL

For SIGEV_SIGNAL, the function zephyr_timer_wrapper() is the handler
between kernel and posix layer.
Here, for one-time timer, reload is equal to 0 and function returns.
As a consequence, handler function was never called.

Signed-off-by: Noemie Gillet <ngillet@sequans.com>