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/common/pico_stdlib/include/pico/stdlib.h

@@ -71,14 +71,14 @@ extern "C" {
  *  PICO_DEFAULT_UART_TX_PIN
  *  PICO_DEFAULT_UART_RX_PIN
  */
-void setup_default_uart();
+void setup_default_uart(void);
 
 /*! \brief Initialise the system clock to 48MHz
  *  \ingroup pico_stdlib
  *
  *  Set the system clock to 48MHz, and set the peripheral clock to match.
  */
-void set_sys_clock_48mhz();
+void set_sys_clock_48mhz(void);
 
 /*! \brief Initialise the system clock
  *  \ingroup pico_stdlib