Add missing includes.

Includes are transitively imported by <string> in String8.h + String16.h
but that include is being removed.

Test: pass
Change-Id: Ide5c011b40b4a4f031dd26ead08b5c8d5d299693
diff --git a/libutils/String16.cpp b/libutils/String16.cpp
index 12bb7c4..ad335c3 100644
--- a/libutils/String16.cpp
+++ b/libutils/String16.cpp
@@ -18,6 +18,8 @@
 
 #include <utils/Log.h>
 
+#include <ctype.h>
+
 #include "SharedBuffer.h"
 
 namespace android {