scanner: Reverse expat/libxml include order

libxml2 unconditonally defines XMLCALL to nothing. Expat does not
redefine XMLCALL if it is already defined, but if it is not, and we are
building with gcc on i386 (not x86-64), it will define it as 'cdecl'.

Including Expat before libxml thus results in a warning about XMLCALL
being redefined. Luckily we can get around this by just reversing the
include order: cdecl is a no-op on Unix-like systems, so by having
libxml first define XMLCALL to nothing and including Expat afterwards,
we avoid the warning and lose nothing.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>