GitLab

Fixes #348

Add the RTC header path in the SDK to the Arduino build path.  Also add
pico_utils to the libpico.a for the datetime_to_str call.

Note that the pico_sdk has a bug in the datetime.h header so you need to
manually bracket the include file as "C":

extern "C" {
    #include "pico/util/datetime.h"
}