perf target: str_error_r() always returns the buffer it receives

perf target: str_error_r() always returns the buffer it receives

So no need for checking if it uses the strerror_r() GNU variant error
reporting mechanism, i.e. if it returns a pointer to a immutable string
internal to glibc.

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: c8b5f2c96d1b ("tools: Introduce str_error_r()")
Link: http://lkml.kernel.org/n/tip-xr83cd4y4r3cn6tq6w4f59jb@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>