string.c: Fix include

string.c needs string.h, otherwise we get:

lib/string.c:730: warning: no previous prototype for 'memdup'

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

lib/string.c

@@ -16,7 +16,7 @@
  */
 
 #include <linux/types.h>
-#include <linux/string.h>
+#include <string.h>
 #include <linux/ctype.h>
 #include <malloc.h>