perf dwarf-aux: Allow unnamed struct/union/enum

perf dwarf-aux: Allow unnamed struct/union/enum

It's possible some struct/union/enum type don't have type name.  Allow
the empty name after "struct"/"union"/"enum" string rather than fail.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230612234102.3909116-2-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>