Add shell interpreter line where missing

The checkbashisms script reports errors like this one:

    script util/check-line-length.sh does not appear to have a #! interpreter line;
    you may get strange results

util/check-categories.sh

@@ -1,3 +1,5 @@
+#!/bin/sh
+
 # Copyright (C) Internet Systems Consortium, Inc. ("ISC")
 #
 # SPDX-License-Identifier: MPL-2.0

util/check-line-length.sh

@@ -1,3 +1,5 @@
+#!/bin/sh
+
 # Copyright (C) Internet Systems Consortium, Inc. ("ISC")
 #
 # SPDX-License-Identifier: MPL-2.0