Avoid C-style cast warnings

hardware/libhardware/include was being included with -isystem, which
hides warnings.  Use a macro to convert modules to hw_device_t** to call
the open function that works in C++ and C.

Bug: 32018017
Test: m -j
Change-Id: If68bf15581975f1217fcab366cef7bc784894641
diff --git a/include/hardware/hardware.h b/include/hardware/hardware.h
index 74f57aa..bf076f6 100644
--- a/include/hardware/hardware.h
+++ b/include/hardware/hardware.h
@@ -201,6 +201,12 @@
 
 } hw_device_t;
 
+#ifdef __cplusplus
+#define TO_HW_DEVICE_T_OPEN(x) reinterpret_cast<struct hw_device_t**>(x)
+#else
+#define TO_HW_DEVICE_T_OPEN(x) (struct hw_device_t**)(x)
+#endif
+
 /**
  * Name of the hal_module_info
  */