Add virtual dtor to LibcurlWrapper.

LibcurlWrapper is deleted but contains no virtual destructor, triggering
warnings with -Wdelete-non-virtual-dtor in clang++.

R=ivanpe@chromium.org

Review URL: https://breakpad.appspot.com/7664002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1337 4c0a9323-5329-0410-9bdc-e9ce6186880e