staging: erofs: fixed -Wmissing-prototype warnings by moving prototypes to header file.

Moved prototypes for two functions to a header file in order to fix the
following warnings:

  drivers/staging/erofs/unzip_vle.c:577:6: warning: no previous prototype for ‘erofs_workgroup_free_rcu’ [-Wmissing-prototypes]
   void erofs_workgroup_free_rcu(struct erofs_workgroup *grp)
        ^~~~~~~~~~~~~~~~~~~~~~~~

  drivers/staging/erofs/unzip_vle.c:1702:5: warning: no previous prototype for ‘z_erofs_map_blocks_iter’ [-Wmissing-prototypes]
   int z_erofs_map_blocks_iter(struct inode *inode,
       ^~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>