perf symbols: Slightly simplify 'err' usage in dso__synthesize_plt_symbols()

Return zero directly instead of needless 'goto out_elf_end' that does
the same thing. That allows 'err' to be initialized to -1 instead of
having to change its value later.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20230120123456.12449-6-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>