Allow building with MBEDTLS_PK_USE_PSA_EC_DATA && MBEDTLS_ECP_C

This isn't officially supported, but it's convenient to build the library
this way for quick testing.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/ssl_tls12_client.c b/library/ssl_tls12_client.c
index 0c5af87..b4b5781 100644
--- a/library/ssl_tls12_client.c
+++ b/library/ssl_tls12_client.c
@@ -2005,9 +2005,9 @@
         return MBEDTLS_ERR_SSL_PK_TYPE_MISMATCH;
     }
 
-#if defined(MBEDTLS_ECP_C)
+#if !defined(MBEDTLS_PK_USE_PSA_EC_DATA)
     const mbedtls_ecp_keypair *peer_key = mbedtls_pk_ec_ro(*peer_pk);
-#endif /* MBEDTLS_ECP_C */
+#endif /* !defined(MBEDTLS_PK_USE_PSA_EC_DATA) */
 
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
     uint16_t tls_id = 0;