Add RTC headers to path and pico_utils to link (#353)

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"
}

lib/platform_inc.txt

@@ -26,6 +26,7 @@
 -iwithprefixbefore/pico-sdk/src/rp2_common/hardware_gpio/include
 -iwithprefixbefore/pico-sdk/src/rp2_common/hardware_i2c/include
 -iwithprefixbefore/pico-sdk/src/rp2_common/hardware_irq/include
+-iwithprefixbefore/pico-sdk/src/rp2_common/hardware_rtc/include
 -iwithprefixbefore/pico-sdk/src/rp2_common/hardware_pio/include
 -iwithprefixbefore/pico-sdk/src/rp2_common/hardware_pll/include
 -iwithprefixbefore/pico-sdk/src/rp2_common/hardware_pwm/include

tools/libpico/CMakeLists.txt

@@ -70,8 +70,9 @@ target_link_libraries(pico
 	pico_standard_link
 	pico_stdlib
 	pico_unique_id
+	pico_util
 	tinyusb
-        tinyusb_device_unmarked
+	tinyusb_device_unmarked
 	pico_audio
 	pico_audio_i2s
 )