softether: fix clock_gettime() linking issue with older glibcs

clock_gettime() is (used to be) provided by librt, so link with it.

Fixes:
http://autobuild.buildroot.net/results/5c1/5c1a1db7c97b5b1cc345daf316d2debde8f0f0b5/
http://autobuild.buildroot.net/results/6a9/6a983bc4ef24358c89474755dcf843a9f01dc1d4/
http://autobuild.buildroot.net/results/f18/f1853ce0f5aafa7ef4b9e3d8ccda50be7ec2c6a7/
http://autobuild.buildroot.net/results/9f8/9f88ab08d07e423fc2f01e6a1bb5e5ae945f02bd/



And many more.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>