mac80211: Fix build errors due to missing string function declarations

Building mac80211 failed, due to missing function declarations in compat/backport-3.15.c
This patch adds the linux/string.h header file to provide these function declarations

Signed-off-by: Maarten Bezemer <maarten.bezemer@gmail.com>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40584 3c298f89-4303-0410-b956-a3cf2f4a3e73