timer: move from common to Unix directory

timer: move from common to Unix directory

EAL common timer doesn't compile under Windows.

Compilation log:
error LNK2019:
unresolved external symbol nanosleep referenced in function
rte_delay_us_sleep
error LNK2019:
unresolved external symbol get_tsc_freq referenced in function set_tsc_freq
error LNK2019:
unresolved external symbol sleep referenced in function set_tsc_freq

The reason was that some functions called POSIX functions.
The solution was to move POSIX dependent functions from common to Unix.

Signed-off-by: Fady Bader <fady@mellanox.com>
Reviewed-by: Tal Shnaiderman <talshn@mellanox.com>
Acked-by: Ranjit Menon <ranjit.menon@intel.com>