util/grub.d/25_bli.in: Fix shebang on unmerged-usr

util/grub.d/25_bli.in: Fix shebang on unmerged-usr

On an unmerged-usr system, grub-mkconfig errors out with the following
error due to /usr/bin/sh not existing:

  /usr/sbin/grub-mkconfig: /etc/grub.d/25_bli: /usr/bin/sh: bad interpreter: No such file or directory

Use a /bin/sh shebang to fix the error as well as match the other
existing files.

Fixes: 158a6583e (util/grub.d/25_bli.in: Activate bli module on EFI)
Signed-off-by: Oskari Pirhonen <xxc3ncoredxx@gmail.com>
Reviewed-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Oliver Steffen <osteffen@redhat.com>