Clean up various C source and headers to appease -Wstrict-prototypes

In C, func() is a function taking an unspecified number of arguments,
vs func(void) a function taking no arguments. In C++ both forms indicate
"no arguments."

Update these headers to use the (void) form, which is correct in both
languages and avoids complaints when -Wstrict-prototypes is specified.

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

@@ -47,6 +47,6 @@ extern stdio_driver_t stdio_usb;
 /*! \brief Explicitly initialize USB stdio and add it to the current set of stdin drivers
  *  \ingroup pico_stdio_uart
  */
-bool stdio_usb_init();
+bool stdio_usb_init(void);
 
 #endif