libnss: fix build with uClibc

libnss: fix build with uClibc

Add a patch defining AT_HWCAP2 locally since uClibc is missing this
macro. Once uClibc updates its elf.h copy we can remove this patch.

Fixes:

  http://autobuild.buildroot.net/results/06f/06f5ee4bc9e623fa08d77278acdcb447148ae997/
  http://autobuild.buildroot.net/results/7cd/7cd7586f5854f9447c37adeedb9c113ba37ebea4/
  http://autobuild.buildroot.net/results/335/335bee755fbec45a6d5336c2501dc2687859ec0e/

Cc: Joseph Kogut <joseph.kogut@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>