libs/utils/energy: gracefully return with a warning in case HWMON is not loaded
libs/utils/energy: gracefully return with a warning in case HWMON is not loaded

Signed-off-by: Patrick Bellasi <431c86f368affbc3148c4669fbd465131f7e82cb@arm.com>

credp/lisa,mdigiorgio/lisa,ARM-software/lisa,credp/lisa,arnoldlu/lisa,arnoldlu/lisa,ARM-software/lisa,credp/lisa,JaviMerino/lisa,bjackman/lisa,mdigiorgio/lisa,credp/lisa,joelagnel/lisa,ARM-software/lisa,joelagnel/lisa,ARM-software/lisa,bjackman/lisa