include: improve compatibility with C++ apps.

This patch lets a C++ application use more of Zephyr by adding guards
and changeing some constructs to the C++11 equivalent.

Changes include:

- Adding guards
- Switching to static_assert
- Switching to a template for ARRAY_SIZE as g++ doesn't have the
  builtin.
- Re-ordering designated initialisers to match the struct field order
  as G++ only supports simple designated initialisers.

Signed-off-by: Michael Hope <mlhx@google.com>

include/usb/class/usb_hid.h

@@ -17,6 +17,10 @@
 #ifndef __USB_HID_H__
 #define __USB_HID_H__
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct usb_hid_class_subdescriptor {
 	u8_t bDescriptorType;
 	u16_t wDescriptorLength;
@@ -153,4 +157,8 @@ void usb_hid_register_device(const u8_t *desc, size_t size,
 /* Initialize USB HID */
 int usb_hid_init(void);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* __USB_HID_H__ */

include/usb/usb_device.h

@@ -39,6 +39,10 @@
 #include <drivers/usb/usb_dc.h>
 #include <usb/usbstruct.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*************************************************************************
  *  USB configuration
  **************************************************************************/
@@ -363,4 +367,8 @@ int usb_transfer_sync(u8_t ep, u8_t *data, size_t dlen, unsigned int flags);
  */
 void usb_cancel_transfer(u8_t ep);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* USB_DEVICE_H_ */