AK: Add missing headers

Example failure:

IDAllocator.h only pulls in AK/Hashtable.h, so any compilation unit that
includes AK/IDAllocator.h without including AK/Traits.h before it used
to be doomed to fail with the cryptic error message "In instantiation of
'AK::HashTable<T, TraitsForT, IsOrdered>::Iterator AK::HashTable<T,
TraitsForT, IsOrdered>::find(const T&) [with T = int; TraitsForT =
AK::Traits: incomplete type 'AK::Traits<int>' used in nested name
specifier".

AK/StringHash.h

@@ -6,6 +6,8 @@
 
 #pragma once
 
+#include <AK/Types.h>
+
 namespace AK {
 
 constexpr u32 string_hash(char const* characters, size_t length)