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>

SVN-Revision: 40584

package/kernel/mac80211/patches/070-add-missing-header.patch

+--- a/compat/backport-3.15.c
++++ b/compat/backport-3.15.c
+@@ -11,6 +11,7 @@
+ #include <linux/kernel.h>
+ #include <linux/device.h>
+ #include <linux/of.h>
++#include <linux/string.h>
+ 
+ /**
+  * devm_kstrdup - Allocate resource managed space and