Only fourteen years have elapsed since "It Should Happen to You," in which Judy Holliday, as Gladys Glover, a nobody who wanted to be a somebody, thought the answer would be to have her name on a billboard?a solution that the movie presented satirically. But our notions of celebrity have changed, and John Brockman, a young mixed-media promoter who has put his face in the television and newspaper ads for "Head," has probably made himself a somebody. Brockman has made himself the star of "Head," though he doesn't even appear in the movie. The depressing possibilities that he has thus opened are sure to be seized on.