Don't use thin archives when building static webkit.

For a static build, we need normal .a files since we're not
going to install .o files. Otherwise we can't use the libs
once the build-dir is cleaned.

Change-Id: I4cbde310cbdb0f24e3397e970a4aea9c8572f6c7
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>