package/libnss: fix PowerPC build failure

NSS assumes <sys/auvx.h> is always present but that's not true, so add a
patch to check if it exists or not.

Fixes:

  http://autobuild.buildroot.net/results/425ba828d30c2bd55ce9f4f00e67bc10d9de2867/



Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>