Build: xml: copy static schema files into build tree

For a VPATH build, dynamically generated schema files will be created in the
build tree, while static schema files remain in the source tree. However,
this breaks running tools (including the cts-scheduler run in "make check")
from the build directory, because Pacemaker expects all schema files to be in
the same location.

I considered making Pacemaker use a schema path rather than a schema directory,
but that seems overcomplicated for such a narrow (and non-production) use case.
Instead, copy all static schema files into the build tree when building the
dynamically generated schemas.