build: skip .png files in format checking

grep reports both whitespace and dos formatting error on .png files, eg.
  File found with trailing whitespace: doc/images/Figure_erasure_code_update.png
  File found with dos formatting: doc/images/Figure_erasure_code_update.png

This patch skips format checking on .png files.

Signed-off-by: Guodong Xu <[email protected]>

Author: docularxu

tools/check_format.sh

@@ -43,6 +43,9 @@ fi
 if hash grep; then
     echo "Checking for dos and whitespace violations..."
     for f in $(git ls-files); do
+	if [[ $f =~ .*png ]]; then
+	    continue
+	fi
 	[ "$verbose" -gt 0 ] && echo "checking whitespace on $f"
 	if grep -q '[[:space:]]$' $f ; then
 	    echo "  File found with trailing whitespace: $f"