Add ifdef for MBEDTLS_ECP_C around ecc_group_to_psa functions

These are unused if MBEDTLS_ECP_C isn't defined.
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 94f6b17..75c7ea8 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -411,6 +411,7 @@
 /* Key management */
 /****************************************************************/
 
+#if defined(MBEDTLS_ECP_C)
 static psa_ecc_curve_t mbedtls_ecc_group_to_psa( mbedtls_ecp_group_id grpid )
 {
     switch( grpid )
@@ -480,6 +481,7 @@
             return( MBEDTLS_ECP_DP_NONE );
     }
 }
+#endif /* defined(MBEDTLS_ECP_C) */
 
 static psa_status_t prepare_raw_data_slot( psa_key_type_t type,
                                            size_t bits,