mesa/main: cast away constness

This avoids a warning about implicitly casting away the constness of the
pointer.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Acked-by: Eric Engestrom <eric@engestrom.ch>

src/mesa/main/marshal.c

@@ -130,7 +130,7 @@ _mesa_unmarshal_ShaderSource(struct gl_context *ctx,
    }
    CALL_ShaderSource(ctx->CurrentServerDispatch,
                      (cmd->shader, cmd->count, string, cmd_length));
-   free(string);
+   free((void *)string);
 }