python,python3: fix underlinking issues with lrt (clock_gettime())

python,python3: fix underlinking issues with lrt (clock_gettime())

Seems it's called underlinking that's happening only
on Ubuntu 12.04 with libressl (that comes from LEDE's
tools folder).

Link here:
https://ubuntuforums.org/showthread.php?t=1870586

I'm still reading about this a bit.
Since I don't really get it.

But applying that fix (as in the link) seems to fix compiling
on Ubuntu 12.04, and tried also on 16.04 (to make sure).

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>