kernel-doc: init kernel version

The kernel-doc script triggers a perl warning when invoked
without KERNELVERSION in the environment, rather make it use
the string "unknown kernel version" instead.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/kernel-doc

@@ -334,7 +334,7 @@ while ($ARGV[0] =~ m/^-(.*)/) {
 
 # get kernel version from env
 sub get_kernel_version() {
-    my $version;
+    my $version = 'unknown kernel version';
 
     if (defined($ENV{'KERNELVERSION'})) {
 	$version = $ENV{'KERNELVERSION'};