Documentation: support glibc versions without htole macros

glibc 2.9 introduced the htole<16/32/64> macros, add them to
tools/include to support older versions of glibc.

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>