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...