tools lib traceevent: Provide le16toh define for older systems

tools lib traceevent: Provide le16toh define for older systems

Where such macro is not present, so just copy its definition from
glibc's endian.h and define it if not already.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/n/tip-4j90i2na07ppidt0z6cbuxr7@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>