Mayor Bill de Blasio’s giant Black Lives Matter mural outside Trump Tower is already getting dumped on. An unidentified man walked up to the giant mural outside the Midtown skyscraper and poured red paint on the bright yellow letters shortly after noon Monday, then ran off before cops showed up to survey the damage. “I knew it was going to happen — there’s all this racist s–t going on,” construction worker Billy McCabe told The Post. “The mural shouldn’t have been done right in front of Trump Tower.” The mural, announced by de Blasio earlier this month, has already become...