filters: Add missing shebangs

The hldiff and plaintext filter scripts are missing their shebangs.
Add those to be correct and consistent.
Additionally, remove the vim comment, it's unnecessary.

Signed-off-by: Tom Schwindl <schwindl@posteo.de>
Acked-by: Moritz Poldrack <moritz@poldrack.dev>

filters/hldiff

@@ -1,4 +1,5 @@
-# vim: set ft=awk :
+#!/usr/bin/awk -f
+
 BEGIN {
 	bright = "\x1B[1m"
 	red = "\x1B[31m"

filters/plaintext

@@ -1,4 +1,5 @@
-# vim: set ft=awk :
+#!/usr/bin/awk -f
+
 BEGIN {
 	dim = "\x1B[2m"
 	cyan = "\x1B[36m"