scripts/xml-preprocess: Make sure this script is invoked via the right Python

If a script is executable and has a shebang line, Meson treats it as
a normal executable, so that this script here is run via the "python3"
binary in the $PATH. However, "python3" might not be in the $PATH at
all, or it might be a wrong version, so we should make sure to run
this script via the Python version that has been chosen for the QEMU
build process. The best way to do this is to remove the executable bit
from the access mode bits. (See also commit 4b424c75)

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1918


Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>