libcurl: fix nss related build failure

Patch is a reduced set from upstream (removed RELEASE-NOTES chunk or it
doesn't apply, cosmetic only). Fixes:
http://autobuild.buildroot.net/results/d0b/d0bf614006e7c7de749dcea7abd584f0aa142418/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>