Kernel: Use Userspace<T> for the execve syscall

Author: https://github.com/bgianfo
Commit: 0f42463eab
Pull-request: https://github.com/SerenityOS/serenity/pull/3062
Reviewed-by: https://github.com/awesomekling

Kernel/API/Syscall.h

@@ -241,7 +241,7 @@ struct ImmutableBufferArgument {
 };
 
 struct StringListArgument {
-    StringArgument* strings { nullptr };
+    Userspace<StringArgument*> strings { };
     size_t length { 0 };
 };