package/libnss: fix build failure with make 4.3.91

Make 4.3.91 doesn't allow to safely override Simple Expanded Variables, so
let's add a patch pending upstream[0] to make those variable Conditional
Expanded.

[0]: https://bugzilla.mozilla.org/show_bug.cgi?id=1801182

Fixes:
http://autobuild.buildroot.net/results/1074143dbea60567cd83be0a23f7c0214d470de9/



Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Tested-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>