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/hardware_clocks/include/hardware/clocks.h

@@ -99,7 +99,7 @@ extern "C" {
  *
  *  Must be called before any other clock function.
  */
-void clocks_init();
+void clocks_init(void);
 
 /*! \brief Configure the specified clock
  *  \ingroup hardware_clocks