ovl: move include of ovl_entry.h into overlayfs.h

Most overlayfs c files already explicitly include ovl_entry.h
to use overlay entry struct definitions and upcoming changes
are going to require even more c files to include this header.

All overlayfs c files include overlayfs.h and overlayfs.h itself
refers to some structs defined in ovl_entry.h, so it seems more
logic to include ovl_entry.h from overlayfs.h than from c files.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>