openipmi: fix missing -lrt for clock_gettime

Add a patch to detect when -lrt is needed. This fixes build with glibc < 2.17.

Fixes:
http://autobuild.buildroot.net/results/2b0/2b0eb7149c7bdd8ebd72c1070aeefb210379a889/
http://autobuild.buildroot.net/results/49f/49fbd7e2dc72279c5b54d0f59a055e3e5e73f648/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>