mkhelp.pl: allow executing this script directly

- Enable execute permission (chmod +x)

- Change interpreter to /usr/bin/env perl

Ref: https://github.com/curl/curl/issues/1743

src/mkhelp.pl

@@ -1,4 +1,4 @@
-#!/usr/local/bin/perl
+#!/usr/bin/env perl
 #***************************************************************************
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 #  Project                     ___| | | |  _ \| |