check-externs.sh: Make output more verbose

Trying to figure out why this is failing on some Debian bots:
https://bugs.freedesktop.org/show_bug.cgi?id=103346

Maybe I should check for GNU grep specifically now that I'm using -n as well.

src/check-externs.sh

@@ -14,7 +14,8 @@ echo 'Checking that all public symbols are exported with HB_EXTERN'
 
 for x in $HBHEADERS; do
 	test -f "$srcdir/$x" && x="$srcdir/$x"
-	$EGREP -B1 '^hb_' "$x" | $EGREP -E -v '^(--|hb_|HB_EXTERN )' -A1
+	$EGREP -B1 -n '^hb_' /dev/null "$x" |
+	$EGREP -v '(^--|:hb_|-HB_EXTERN )' -A1
 done |
 grep . >&2 && stat=1