Add missing backslash for shellcheck

What does this Merge Request do?

This MR fixes a bug where the support/ci/shellcheck didn't actually run correct because of a missing \.

@paisleyrob picked this up at !1320 (comment 362257437).

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.