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/include/pico/stdio.h

@@ -51,7 +51,7 @@ typedef struct stdio_driver stdio_driver_t;
  *
  * \see stdio_uart, stdio_usb, stdio_semihosting
  */
-void stdio_init_all();
+void stdio_init_all(void);
 
 /*! \brief Initialize all of the present standard stdio types that are linked into the binary.
  * \ingroup pico_stdio
@@ -61,7 +61,7 @@ void stdio_init_all();
  *
  * \see stdio_uart, stdio_usb, stdio_semihosting
  */
-void stdio_flush();
+void stdio_flush(void);
 
 /*! \brief Return a character from stdin if there is one available within a timeout
  * \ingroup pico_stdio

src/rp2_common/pico_stdio/include/pico/stdio/driver.h

@@ -12,7 +12,7 @@
 
 struct stdio_driver {
     void (*out_chars)(const char *buf, int len);
-    void (*out_flush)();
+    void (*out_flush)(void);
     int (*in_chars)(char *buf, int len);
     stdio_driver_t *next;
 #if PICO_STDIO_ENABLE_CRLF_SUPPORT

src/rp2_common/pico_stdio/stdio.c

@@ -33,7 +33,7 @@ static stdio_driver_t *filter;
 #if PICO_STDOUT_MUTEX
 auto_init_mutex(print_mutex);
 
-bool stdout_serialize_begin() {
+bool stdout_serialize_begin(void) {
     int core_num = get_core_num();
     uint32_t owner;
     if (!mutex_try_enter(&print_mutex, &owner)) {
@@ -46,15 +46,15 @@ bool stdout_serialize_begin() {
     return true;
 }
 
-void stdout_serialize_end() {
+void stdout_serialize_end(void) {
     mutex_exit(&print_mutex);
 }
 
 #else
-static bool print_serialize_begin() {
+static bool print_serialize_begin(void) {
     return true;
 }
-static void print_serialize_end() {
+static void print_serialize_end(void) {
 }
 #endif
 
@@ -255,7 +255,7 @@ void stdio_init_all() {
 #endif
 }
 
-int WRAPPER_FUNC(getchar)() {
+int WRAPPER_FUNC(getchar)(void) {
     char buf[1];
     if (0 == stdio_get_until(buf, sizeof(buf), at_the_end_of_time)) {
         return PICO_ERROR_TIMEOUT;