Cross building it on Ubuntu 16.04 to ARM ends up showing we get
the free() prototype by luck in other environments, fix it.

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-0ktfgmmyhcfw8ondka2013f3@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>