perf tools: Fix build error due to zfree() cast

perf tools: Fix build error due to zfree() cast

It failed to build perf on my ubuntu 10.04 box (gcc 4.4.3):

    CC       util/strlist.o
  cc1: warnings being treated as errors
  util/strlist.c: In function ‘str_node__delete’:
  util/strlist.c:42: error: dereferencing type-punned pointer will break strict-aliasing rules
  util/strlist.c:42: error: dereferencing type-punned pointer will break strict-aliasing rules
    CC       util/strfilter.o
  make: *** [util/strlist.o] Error 1

    CC       util/srcline.o
  cc1: warnings being treated as errors
  util/srcline.c: In function ‘addr2line_init’:
  util/srcline.c:132: error: dereferencing type-punned pointer will break strict-aliasing rules
  util/srcline.c:132: error: dereferencing type-punned pointer will break strict-aliasing rules
  util/srcline.c: In function ‘addr2line_cleanup’:
  util/srcline.c:143: error: dereferencing type-punned pointer will break strict-aliasing rules
  util/srcline.c:143: error: dereferencing type-punned pointer will break strict-aliasing rules
  make: *** [util/srcline.o] Error 1

It seems it only allows to remove 'const' qualifier.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1389276479-9047-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>