Include <strings.h> for ffs.

No pun intended.

Change-Id: I89b2c73af143053223f8116e65456f85f09fb591
diff --git a/keystore/keystore.cpp b/keystore/keystore.cpp
index e56edfd..9ee2598 100644
--- a/keystore/keystore.cpp
+++ b/keystore/keystore.cpp
@@ -20,6 +20,7 @@
 #include <stdio.h>
 #include <stdint.h>
 #include <string.h>
+#include <strings.h>
 #include <unistd.h>
 #include <signal.h>
 #include <errno.h>