doc lint: Perl "strict" and "warnings" in lint-gitlink.perl

Amend this script added in ab81411ced (ci: validate "linkgit:" in
documentation, 2016-05-04) to pass under "use strict", and add a "use
warnings" for good measure.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/lint-gitlink.perl

@@ -1,5 +1,7 @@
 #!/usr/bin/perl
 
+use strict;
+use warnings;
 use File::Find;
 use Getopt::Long;
 
@@ -45,7 +47,7 @@ sub lint {
 				report($where, $target, "no such source");
 				next;
 			}
-			$real_section = grab_section($page);
+			my $real_section = grab_section($page);
 			if ($real_section != $section) {
 				report($where, $target,
 					"wrong section (should be $real_section)");