perf test evlist-open-close: Use inline func to convert timeval to usec

perf test evlist-open-close: Use inline func to convert timeval to usec

This patch introduces a new inline function to convert a timeval to
usec.

This function will be used also in the next patch.

Signed-off-by: Riccardo Mancini <rickyman7@gmail.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/b95035ec4a125355be8ea843f7275c4580da6398.1629490974.git.rickyman7@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>