fix-encoding-args.pl: Don't use a feature available only starting with perl 5.14

Specifically: Window msys git uses an earlier version of Perl (5.8) and thus
	      invoking fix-encoding-args.pl in the pre-commit hook script
	      gives a Perl syntax error.

See https://code.wireshark.org/review/4172 for details

(Also see following for Perl info:
 http://www.perl.com/pub/2011/05/new-features-of-perl-514-non-destructive-substitution.html)

Change-Id: I95a222eaa31942cc6e873ffbde57c5fa5bd2a625
Reviewed-on: https://code.wireshark.org/review/4245
Reviewed-by: Bill Meier <wmeier@newsguy.com>