timeval: Import ctypes Python library within a try statement.

Older versions of Python do not have ctypes as a default installed
package. This patch puts the 'import ctypes' statement inside a try
statement.

This fixes a bug introduced by commit 8396f (timeval: Use monotonic
time in OVS Python timeval library).

Signed-off-by: Ryan Wilson <wryan@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>