openipmi: needs NPTL support

openipmi uses the pthread_condattr_setclock() function, which is only
available with the NPTL thread implementation.

Fixes:

  http://autobuild.buildroot.net/results/fc4084f8d1ff3fe98066c7cc051c2127bbe77ec5/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>