ChromeOS: add missing string.h in gnvs.c

string.h is required to build with the reference toolchain.

Change-Id: I9fd8d2ea8fc676d3502989cbcc7aefe3b2d738b6
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/935
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/vendorcode/google/chromeos/gnvs.c

@@ -18,6 +18,7 @@
  */
 
 #include <types.h>
+#include <string.h>
 #include <cbfs.h>
 #include <console/console.h>
 #include "gnvs.h"