cmake: Make SVNVersion.inc step depend on GetSVN.cmake.

This way, the step generating SVNVersion.inc gets rerun every time someone
changes GetSVN.cmake (which is the file that decides how the contents of
SVNVersion.inc look). This makes hacking on GetSVN.cmake a bit easier.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223861 91177308-0d34-0410-b5e6-96231b3b80d8