python3: don't use wcsftime() on uClibc

As investigated in bug #7646, wcsftime() doesn't work properly with
uClibc. Until it gets fixed in uClibc, let's tell Python 3 to not use
it. Python 3 will fall back to strftime(), which works properly.

[Peter: fix typo in comment]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Beyonlo <beyonlo@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>