libnspr: fix static build

Disable shared library in a static build.

Fixes:
http://autobuild.buildroot.net/results/6ff/6ffae151ec3eaca175923ab860cf42167871164c/
http://autobuild.buildroot.net/results/cab/cab19721c43b3256dd51ae511016bc9ea440512a/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>