Add missing extern "C" to header files (closes #104)

src/rp2_common/pico_stdio_usb/include/pico/stdio_usb.h

@@ -42,6 +42,10 @@
 #define PICO_STDIO_USB_LOW_PRIORITY_IRQ 31
 #endif
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 extern stdio_driver_t stdio_usb;
 
 /*! \brief Explicitly initialize USB stdio and add it to the current set of stdin drivers
@@ -49,4 +53,8 @@ extern stdio_driver_t stdio_usb;
  */
 bool stdio_usb_init(void);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif