Prevent function not used compilation error

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>

library/ssl_tls13_keys.c

@@ -136,6 +136,8 @@ static void ssl_tls13_hkdf_encode_label(
     *dst_len = total_hkdf_lbl_len;
 }
 
+#if defined( MBEDTLS_TEST_HOOKS )
+
 MBEDTLS_STATIC_TESTABLE
 psa_status_t mbedtls_psa_hkdf_expand( psa_algorithm_t alg,
                                       const unsigned char *prk, size_t prk_len,
@@ -255,6 +257,8 @@ cleanup:
     return( ret );
 }
 
+#endif /* MBEDTLS_TEST_HOOKS */
+
 int mbedtls_ssl_tls13_hkdf_expand_label(
                      mbedtls_md_type_t hash_alg,
                      const unsigned char *secret, size_t secret_len,