perf trace: Enclose strings with double quotes

To match 'strace' output, helping with upcoming regression tests
comparing both outputs.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-jab52t1dcuh6vlztqle9g7u9@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>