perf util: Make copyfile_offset() global

perf util: Make copyfile_offset() global

It will be used outside of util object in following patches.

Committer note:

We need to have the header with the definition for loff_t in util.h
since we now use it in the copyfile_offset() signature.

Also move that prototype closer to the other copyfile_ prefixed
functions.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180913125450.21342-10-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>