perf tools: Make copyfile_offset() static

perf tools: Make copyfile_offset() static

There are no usage outside util.c and this is the only remaining reason
for fcntl.h to be included in util.h, to get the loff_t definition in
Alpine Linux, so make it static.

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-2dzlsao7k6ihozs5karw6kpx@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>