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_interp/include/hardware/interp.h

@@ -231,7 +231,7 @@ static inline void interp_config_set_force_bits(interp_config *c, uint bits) {
  *
  * \return A default interpolation configuration
  */
-static inline interp_config interp_default_config() {
+static inline interp_config interp_default_config(void) {
     interp_config c = {0};
     // Just pass through everything
     interp_config_set_mask(&c, 0, 31);