AK: Define both ::nullptr_t and std::nullptr_t

LLVM 15 switched around what it's basing its `nullptr_t` definitions on,
it's now defining `std::nullptr_t` using `::nullptr_t` instead of the
other way around.

Work around any errors that result from that by just defining it both in
the global namespace as well as in `std` ourselves.

AK/Types.h

@@ -65,6 +65,8 @@ namespace std { // NOLINT(cert-dcl58-cpp) nullptr_t must be in ::std:: for some
 using nullptr_t = decltype(nullptr);
 }
 
+using nullptr_t = std::nullptr_t;
+
 static constexpr FlatPtr explode_byte(u8 b)
 {
     FlatPtr value = b;