Exclude .svn (and other "dot" directories) when installing headers.

My fix for PR15820 in r180132 inadvertently removed the exclusion for ".*".
This puts it back again. Thanks for Nico Weber for pointing this out!

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@203807 91177308-0d34-0410-b5e6-96231b3b80d8