Document that ECP_RESTARTABLE depends on ECP_C

This is not new, it had always been the case, just not documented.

Pointed out by depends.py pkalgs (again, now that restartable is part of
full).

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index 921a749..5d4033c 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -723,6 +723,8 @@
  *
  * \note  This option only works for Short Weierstrass curves.
  *
+ * Requires: MBEDTLS_ECP_C
+ *
  * Uncomment this macro to enable restartable ECC computations.
  */
 //#define MBEDTLS_ECP_RESTARTABLE