perf intel pt decoder: clang has no -Wno-override-init

perf intel pt decoder: clang has no -Wno-override-init

So set it only for other compilers, allowing us to overcome yet another
build failure due to an inexistent clang -W option:

  error: unknown warning option '-Wno-override-init'; did you mean '-Wno-override-module'? [-Werror,-Wunknown-warning-option]

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-oaa1ici3j8nygp4pzl2oobh3@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>