Touch up checkformat.bash

The logic can be slightly simplified,
and the script doesn't require Bash after all.

contrib/checkformat.sh

@@ -1,10 +1,10 @@
-#!/usr/bin/env bash
+#!/usr/bin/env sh
 
 # SPDX-License-Identifier: MIT
 
 clang-format --version
 
-find . -type f \( -iname '*.hpp' -o -iname '*.cpp' \) -exec clang-format -i {} +
+find . -type f -iname '*.[ch]pp' -exec clang-format -i {} +
 
 if ! git diff-index --quiet HEAD --; then
 	echo 'Unformatted files:'