NSS: Do not use deprecated header files
    
    In file included from src/util/cert/nss/cert.c:26:
    /usr/include/nss3/key.h:9:9: note: #pragma message: key.h is deprecated. Please include keyhi.h instead.
     #pragma message("key.h is deprecated. Please include keyhi.h instead.")
             ^~~~~~~
    
    Merges: https://pagure.io/SSSD/sssd/pull-request/3930
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>