* pico_stdio_usb: be more explicit about includes, fix warning (#257)
* pico_base: NDEBUG backwards for absolute_time_t (#255)
* pico_util: missing extern C in queue.h (#249)
* build: remove -march which was masking -mcpu, now SVC available (#253)
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.
