staging: erofs: fixed -Wmissing-prototype warnings by making functions static.

Define three functions which are not used outside their own
translation-units as static in order to fix the following warnings:

  drivers/staging/erofs/utils.c:134:6: warning: no previous prototype for ‘erofs_try_to_release_workgroup’ [-Wmissing-prototypes]
   bool erofs_try_to_release_workgroup(struct erofs_sb_info *sbi,
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  drivers/staging/erofs/unzip_vle.c:592:6: warning: no previous prototype for ‘z_erofs_vle_work_release’ [-Wmissing-prototypes]
   void z_erofs_vle_work_release(struct z_erofs_vle_work *work)
        ^~~~~~~~~~~~~~~~~~~~~~~~

  drivers/staging/erofs/unzip_vle_lz4.c:16:5: warning: no previous prototype for ‘z_erofs_unzip_lz4’ [-Wmissing-prototypes]
   int z_erofs_unzip_lz4(void *in, void *out, size_t inlen, size_t outlen)
       ^~~~~~~~~~~~~~~~~

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