perf trace: Add missing ' = ' in the default formatting of syscall returns

We lost it recently, put it back.

Before:

  789.499 ( 0.001 ms): libvirtd/1175 lseek(fd: 22, whence: CUR) 4328

After:

  789.499 ( 0.001 ms): libvirtd/1175 lseek(fd: 22, whence: CUR) = 4328

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>
Fixes: 1f63139c ("perf trace beauty: Simplify syscall return formatting")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>