libnspr: fix build issue on ARM with certain toolchains

Fixes issues like
http://autobuild.buildroot.org/results/aed444bf37b9e289866a9b6c754ed74a3cc0f3b2/build-end.log

.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>