tools lib traceevent plugin function: Initialize 'index' variable

Detected with clang:

    CC       /tmp/build/perf/plugin_function.o
  plugin_function.c:145:6: warning: variable 'index' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
          if (parent && ftrace_indent->set)
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
  plugin_function.c:148:29: note: uninitialized use occurs here
          trace_seq_printf(s, "%*s", index*3, "");
                                     ^~~~~
  plugin_function.c:145:2: note: remove the 'if' if its condition is always true
          if (parent && ftrace_indent->set)
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  plugin_function.c:145:6: warning: variable 'index' is used uninitialized whenever '&&' condition is false [-Wsometimes-uninitialized]
          if (parent && ftrace_indent->set)
              ^~~~~~
  plugin_function.c:148:29: note: uninitialized use occurs here
          trace_seq_printf(s, "%*s", index*3, "");
                                     ^~~~~
  plugin_function.c:145:6: note: remove the '&&' if its condition is always true
          if (parent && ftrace_indent->set)
              ^~~~~~~~~
  plugin_function.c:133:11: note: initialize the variable 'index' to silence this warning
          int index;
                   ^
                    = 0
  2 warnings generated.

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-b5wyjocel55gorl2jq2cbxrr@git.kernel.org


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