perf trace: Add a prefix member to the strarray class

perf trace: Add a prefix member to the strarray class

So that the user, in an upcoming patch, can select printing it to get
the full string as used in the source code, not one with a common prefix
chopped off so as to make the output more compact.

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-zypczc88gzbmeqx7b372s138@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>