perf tools: Add missing header to color.c

It uses isatty(), so needs unistd.h, include it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-ivwuz8f68tb3sdcpguo9wmvx@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>