libnspr: fix build with ARMv7 toolchains

Fixes the following build failure:

  http://autobuild.buildroot.org/results/d7323831372050e425a34f5104a46d8cbd6be214/build-end.log



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