libnspr: needs dynamic library support

The only package selecting libnspr (libnss) requires dynamic library already.

Fixes:
http://autobuild.buildroot.net/results/296/296bc0fd888a43700d2ea020f8486932c5d37760/
http://autobuild.buildroot.net/results/a65/a65588aa91a87f9a3d8af2a485de2df3e30cfc19/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>