xl2tpd: fix warnings with gcc 5

When compile with gcc 5.x, xl2tpd complains warnings:

| misc.h:68:20: warning: inline function 'swaps' declared but never
defined
|  extern inline void swaps (void *, int);
|                      ^

Backport patch to fix it.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>