expat: Don't use getrandom() in the -native case

expat: Don't use getrandom() in the -native case

getrandom() is only available in glibc 2.25+ and uninative may relocate
binaries onto systems that don't have this function. For now, force
the code to the older codepath until we can come up with a better solution
for this kind of issue.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>