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>
diff --git a/scripts/kernel-doc b/scripts/kernel-doc
index e4fa8d9..8cf528b 100755
--- a/scripts/kernel-doc
+++ b/scripts/kernel-doc
@@ -334,7 +334,7 @@
 
 # get kernel version from env
 sub get_kernel_version() {
-    my $version;
+    my $version = 'unknown kernel version';
 
     if (defined($ENV{'KERNELVERSION'})) {
 	$version = $ENV{'KERNELVERSION'};