perf tools: Add missing util.h to pick up 'page_size' variable

Not to depend of getting it indirectly.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-tirjsmvu4ektw0k7lm8k9lhu@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>