Add missing <string.h> include.

Change-Id: Ia9fdc3516343a1fbfa5cba6fb296626de3f3abd9
diff --git a/keystore/keyblob_utils.cpp b/keystore/keyblob_utils.cpp
index b208073..9970e8c 100644
--- a/keystore/keyblob_utils.cpp
+++ b/keystore/keyblob_utils.cpp
@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#include <string.h>
 #include <sys/types.h>
 #include <unistd.h>