package/expat: bump to version 2.4.6

This release fixes a regression introduced by one of the security fixes
in 2.4.5.

https://blog.hartwork.org/posts/expat-2-4-6-released
https://github.com/libexpat/libexpat/blob/R_2_4_6/expat/Changes



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>