buildroot

package/openpowerlink: fix uClibc build

In the demo application code, va_list type is used in eventlogstring.h
so stdarg.h must be included to define it.

This problem occurs with a uClibc-ng based toolchain.

Fixes:
http://autobuild.buildroot.net/results/a9e/a9e7615a19922706039bf97ccb94bcf5b99330b2

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>