in Android NDK, UNIX_PATH_MAX is already defined,
add ifndef to check.

linux/un.h:
22: #define UNIX_PATH_MAX 108

Cc: Chen Hu <hu1.chen@intel.com>
Signed-off-by: Zhang Ning <ning.a.zhang@intel.com>
Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com>