AK: Add a forward declaration header

You can now #include <AK/Forward.h> to get most of the AK types as
forward declarations.

Header dependency explosion is one of the main contributors to compile
times at the moment, so this is a step towards smaller include graphs.

Libraries/LibC/syslog.cpp

@@ -27,6 +27,7 @@
 // Has to be defined before including due to legacy Unices
 #define SYSLOG_NAMES 1
 
+#include <AK/String.h>
 #include <AK/StringBuilder.h>
 #include <stdio.h>
 #include <string.h>