AK: Include HashTable.h in StringImpl.cpp

This was used without an include, I'm not sure how it didn't break
before :P

AK/StringImpl.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <AK/FlyString.h>
+#include <AK/HashTable.h>
 #include <AK/Memory.h>
 #include <AK/StdLibExtras.h>
 #include <AK/StringHash.h>