security/nss: fix build with GCC architectures

-Werror=unused-function causes:
verified/kremlin/kremlib/dist/minimal/FStar_UInt128.h:82:37: error: 'FStar_UInt128_mul32' declared 'static' but never defined [-Werror=unused-function]

82 | inline static FStar_UInt128_uint128 FStar_UInt128_mul32(uint64_t x, uint32_t y);

Add -Wno-error=unused-function to fix build.

Approved by: portmgr (ports tree compliance)
MFH: 2020Q1 (ports tree compliance)