rules: filter out irrelevant files

rules: filter out irrelevant files

It's often handy to make executables depend on each other, e.g. make a
test depend on a helper. This doesn't work now, as linker
will attempt to use the helper as an object.
To fix, filter only relevant file types before linking an executable.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>