py/mpconfig.h: Define MICROPY_PY_USSL_FINALISER only if not defined.

So a port can define it even if MICROPY_PY_USSL is not defined.
Signed-off-by: Damien George <damien@micropython.org>

py/mpconfig.h

@@ -1552,7 +1552,10 @@ typedef double mp_float_t;
 
 #ifndef MICROPY_PY_USSL
 #define MICROPY_PY_USSL (0)
+#endif
+
 // Whether to add finaliser code to ussl objects
+#ifndef MICROPY_PY_USSL_FINALISER
 #define MICROPY_PY_USSL_FINALISER (0)
 #endif