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_semihosting/include/pico/stdio_semihosting.h

@@ -29,6 +29,6 @@ extern stdio_driver_t stdio_semihosting;
  *
  * \note this method is automatically called by \ref stdio_init_all() if `pico_stdio_semihosting` is included in the build
  */
-void stdio_semihosting_init();
+void stdio_semihosting_init(void);
 
 #endif