kernel-doc: Make it compatible with Perl versions below 5.12 again

Changeset 4d732701('scripts/kernel-doc: Replacing highlights
hash by an array') broke compatibility of the kernel-doc script with
older versions of perl by using "keys ARRAY" syntax with is available
only on Perl 5.12 or newer, according with:
	http://perldoc.perl.org/functions/keys.html

Restore backward compatibility by replacing "foreach my $k (keys ARRAY)"
by a C-like variant: "for (my $k = 0; $k < !ARRAY; $k++)"
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/kernel-doc

@@ -2711,7 +2711,7 @@ $kernelversion = get_kernel_version();
 
 # generate a sequence of code that will splice in highlighting information
 # using the s// operator.
-foreach my $k (keys @highlights) {
+for (my $k = 0; $k < @highlights; $k++) {
     my $pattern = $highlights[$k][0];
     my $result = $highlights[$k][1];
 #   print STDERR "scanning pattern:$pattern, highlight:($result)\n";