How can I get statistics about how much (in percents) the Linux kernel source code changes in one year?
What you are looking for can be found on the Ohloh website, which by the way indexes the Linux GIT repository. There you will see a graph showing you how much the kernel has changed over 1 yr, 3 yrs, 5 yrs, 10 yrs or All. By default it will show you the statistics for the source code but you can also get statistics about Languages, Committers, Commits. You can then manually calculate the change %. The change in source code between 2010 and 2011 is up 11.4%.
Your best bet would be to get statistical data about the commits themselves.
gitstats is designed for statistical analysis of Git repositories (which is the VCS that Linux uses), and should fit your needs.
You'll find this kind of information on lwn.net website. It's made by Jonathan Corbet and has an in-depth coverage of Linux kernel. Jonathan Corbet is a highly-recognized contributor to the Linux kernel community.
On the Kernel Index page, you can look at "who wrote ..." pages. They contains detailled information about changes in linux kernel from a version to another.
Edit: found an official source.