firefox: add nss libraries to PRIVATE_LIBS and add dependency on external nss, nspr

* fixes following warnings and makes shlibs providers for nss libs
  more deterministic
  WARNING: firefox-10.0.11esr is already registered as shlib provider for libfreebl3.so, ignoring nss-3.13.3 trying to register the same
  WARNING: firefox-10.0.11esr is already registered as shlib provider for libnss3.so, ignoring nss-3.13.3 trying to register the same
  WARNING: firefox-10.0.11esr is already registered as shlib provider for libnssckbi.so, ignoring nss-3.13.3 trying to register the same
  WARNING: firefox-10.0.11esr is already registered as shlib provider for libsmime3.so, ignoring nss-3.13.3 trying to register the same
  WARNING: firefox-10.0.11esr is already registered as shlib provider for libnssutil3.so, ignoring nss-3.13.3 trying to register the same
  WARNING: firefox-10.0.11esr is already registered as shlib provider for libnssdbm3.so, ignoring nss-3.13.3 trying to register the same
  WARNING: firefox-10.0.11esr is already registered as shlib provider for libssl3.so, ignoring nss-3.13.3 trying to register the same
  WARNING: firefox-10.0.11esr is already registered as shlib provider for libsoftokn3.so, ignoring nss-3.13.3 trying to register the same

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>