Explicitly include defaults.cc in the shared_library gyp target.

Explicitly include defaults.cc in the shared_library gyp target, otherwise
the whole compilation unit is stripped by the Windows linker, since none of it's
symbols are referenced by any other compilation unit in V8.

BUG=None
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/25994002

Patch from Ross McIlroy <rmcilroy@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17107 ce2b1a6d-e550-0410-aec6-3dcde31c8c00