LibCore+AK: Move MappedFile from AK to LibCore

MappedFile is strictly a userspace thing, so it doesn't belong in AK
(which is supposed to be user/kernel agnostic.)

Userland/Libraries/LibCore/CMakeLists.txt

@@ -16,6 +16,7 @@ set(SOURCES
     LocalServer.cpp
     LocalSocket.cpp
     LockFile.cpp
+    MappedFile.cpp
     MimeData.cpp
     NetworkJob.cpp
     NetworkResponse.cpp