p11_child: fix initializer error
    
    Building with:
    ```
    $ echo $CFLAGS
    -m64 -mtune=generic -fstack-protector-all -Wall -Wextra -Wno-sign-compare -Wshadow -Wunused-variable -Wno-unused-parameter -Wno-error=cpp -O0 -ggdb3 -Werror -Wp,-U_FORTIFY_SOURCE
    ```
    
    Produces:
    ```
    /home/pbrezina/workspace/sssd/src/p11_child/p11_child_openssl.c: In function ‘get_preferred_rsa_mechanism’:
    /home/pbrezina/workspace/sssd/src/p11_child/p11_child_openssl.c:1296:9: error: missing initializer for field ‘evp_md’ of ‘struct prefs’ [-Werror=missing-field-initializers]
     1296 |         { 0, NULL }
          |         ^
    /home/pbrezina/workspace/sssd/src/p11_child/p11_child_openssl.c:1288:23: note: ‘evp_md’ declared here
     1288 |         const EVP_MD *evp_md;
          |                       ^~~~~~
    ```
    
    Reviewed-by: Alexey Tikhonov <atikhonov@redhat.com>