libxml2: only use static output with both shared & static

The static output should only get created when both enableShared and
enableStatic are set. Otherwise there would be libraries missing from
the main output when enableShared = false & enableStatic = true. This
can cause issues in some packages that don’t know about libxml2’s
static output.

(cherry picked from commit 2bd6bb0a4bf21005d8877c735709cd21d22e05bd)
(cherry picked from commit 1421a39c1e62584d346185ad49484b11b7703dc1)