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

src/rp2_common/pico_stdio_uart/include/pico/stdio_uart.h

@@ -23,6 +23,10 @@
 #define PICO_STDIO_UART_DEFAULT_CRLF PICO_STDIO_DEFAULT_CRLF
 #endif
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 extern stdio_driver_t stdio_uart;
 
 /*! \brief Explicitly initialize stdin/stdout over UART and add it to the current set of stdin/stdout drivers
@@ -59,4 +63,8 @@ void stdin_uart_init(void);
  */
 void stdio_uart_init_full(uart_inst_t *uart, uint baud_rate, int tx_pin, int rx_pin);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif