Kernel: Move File.{cpp,h} into FileSystem/

Also tweak the kernel's Makefile to use -nostdinc and -nostdinc++.
This prevents us from picking up random headers from ../Root, which may
include older versions of kernel headers.

Since we still need <initializer_list> for Vector, we specifically include
the necessary GCC path. This is a bit hackish but it works for now.

Kernel/FileSystem/File.cpp

@@ -0,0 +1,30 @@
+#include <Kernel/FileSystem/File.h>
+#include <Kernel/FileSystem/FileDescription.h>
+
+File::File()
+{
+}
+
+File::~File()
+{
+}
+
+KResultOr<NonnullRefPtr<FileDescription>> File::open(int options)
+{
+    UNUSED_PARAM(options);
+    return FileDescription::create(this);
+}
+
+void File::close()
+{
+}
+
+int File::ioctl(FileDescription&, unsigned, unsigned)
+{
+    return -ENOTTY;
+}
+
+KResultOr<Region*> File::mmap(Process&, FileDescription&, VirtualAddress, size_t, size_t, int)
+{
+    return KResult(-ENODEV);
+}