ssl_server2.c: fix build err (key_slot - unused variable)

Signed-off-by: Przemyslaw Stekiel <przemyslaw.stekiel@mobica.com>
diff --git a/programs/ssl/ssl_server2.c b/programs/ssl/ssl_server2.c
index f0d021d..05cf085 100644
--- a/programs/ssl/ssl_server2.c
+++ b/programs/ssl/ssl_server2.c
@@ -1320,7 +1320,7 @@
     mbedtls_pk_context pkey;
     mbedtls_x509_crt srvcert2;
     mbedtls_pk_context pkey2;
-#if defined(MBEDTLS_USE_PSA_CRYPTO)
+#if defined(MBEDTLS_ECDSA_C) && defined(MBEDTLS_USE_PSA_CRYPTO)
     psa_key_id_t key_slot = 0; /* invalid key slot */
 #endif
     int key_cert_init = 0, key_cert_init2 = 0;