tools lib traceevent: Use 'const' in variables pointing to const strings

tools lib traceevent: Use 'const' in variables pointing to const strings

Fixing the build on fedora 14, 32-bit:

  tools/lib/traceevent/event-parse.c: In function ‘find_cmdline’:
  tools/lib/traceevent/event-parse.c:183:3: error: return discards qualifiers from pointer target type
  tools/lib/traceevent/event-parse.c:186:3: error: return discards qualifiers from pointer target type
  tools/lib/traceevent/event-parse.c:195:2: error: return discards qualifiers from pointer target type
  tools/lib/traceevent/event-parse.c: In function ‘process_func_handler’:
  tools/lib/traceevent/event-parse.c:2658:9: error: assignment discards qualifiers from pointer target type
  tools/lib/traceevent/event-parse.c:2660:9: error: assignment discards qualifiers from pointer target type
  tools/lib/traceevent/event-parse.c: In function ‘print_mac_arg’:
  tools/lib/traceevent/event-parse.c:3892:14: error: initialization discards qualifiers from pointer target type
  tools/lib/traceevent/event-parse.c:3906:7: error: assignment discards qualifiers from pointer target type
  tools/lib/traceevent/event-parse.c: In function ‘pevent_print_event’:
  tools/lib/traceevent/event-parse.c:4412:24: error: initialization discards qualifiers from pointer target type

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/n/tip-0k5g8urwu7vwkgbcbt2x05fe@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>