perf trace: Add the syscall_arg_fmt pointer to syscall_arg

So that the scnprintf beautifiers can access it, as will be the case
with the char array one in the following csets, that needs to know
the number of elements in an array.

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>
Link: https://lkml.kernel.org/n/tip-01qmjqv6cb1nj1qy4khdexce@git.kernel.org


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