configure: move QEMU_INCLUDES to meson

Confusingly, QEMU_INCLUDES is not used by configure tests.  Moving
it to meson.build ensures that Windows paths are specified instead of
the msys paths like /c/Users/...

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>