util/bash-completion: Disable SC2120 shellcheck warning

SC2120 (warning): function references arguments, but none are ever passed.

In grub-completion.bash.in line 63:
__grub_get_options_from_help () {
^-- SC2120 (warning)
     local prog

     if [ $# -ge 1 ]; then
         prog="$1"

The arg of __grub_get_options_from_help() is optional. So, the current
code meets the exception and does not need to be modified. Ignoring the
warning then.

More: https://github.com/koalaman/shellcheck/wiki/SC2120

Signed-off-by: t.feng <fengtao40@huawei.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

Author: t.feng

util/bash-completion.d/grub-completion.bash.in

@@ -63,6 +63,7 @@ __grubcomp () {
 
 # Function that return long options from the help of the command
 # - arg: $1 (optional) command to get the long options from
+# shellcheck disable=SC2120
 __grub_get_options_from_help () {
      local prog