backports: backport-3.15: Include fix

In this version the correct linux string.h is included.

Include string.h to avoid compile issues if warnings are treated as errors:

backports-3.14-1/compat/backport-3.15.c: In function 'backport_devm_kstrdup':
backports-3.14-1/compat/backport-3.15.c:33:2: error: implicit declaration of function 'strlen' [-Werror=implicit-function-declaration]
size = strlen(s) + 1;
^
backports-3.14-1/compat/backport-3.15.c:33:9: warning: incompatible implicit declaration of built-in function 'strlen' [enabled by default]

   size = strlen(s) + 1;
          ^
backports-3.14-1/compat/backport-3.15.c:36:3: error: implicit declaration of function 'memcpy' [-Werror=implicit-function-declaration]
    memcpy(buf, s, size);
    ^

backports-3.14-1/compat/backport-3.15.c:36:3: warning: incompatible implicit declaration of built-in function 'memcpy' [enabled by default]

---

@@ -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