limits.h: add define for HOST_NAME_MAX

according to the rules defined here:
    http://pubs.opengroup.org/onlinepubs/009695399/basedefs/limits.h.html
add the definition for HOST_NAME_MAX to limits.h file,
and set the default value to _POSIX_HOST_NAME_MAX as 255

Change-Id: Iddd5c6c569f4e0a14994c7a7c54985f3e7809fc4
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
diff --git a/libc/include/limits.h b/libc/include/limits.h
index 69df082..6ae629b 100644
--- a/libc/include/limits.h
+++ b/libc/include/limits.h
@@ -97,4 +97,5 @@
 /* POSIX says these belong in <unistd.h> but BSD has some in <limits.h>. */
 #include <machine/posix_limits.h>
 
+#define HOST_NAME_MAX _POSIX_HOST_NAME_MAX
 #endif /* !_LIMITS_H_ */