[Upstream] depends: expat 2.2.7

[lyricidal]

Major changes in expat 2.2.7:

• [CVE-2018-20843] 88k xml file uses >2G memory libexpat/libexpat#186 xmlparse.c: Fix extraction of namespace prefix from XML name (#186) libexpat/libexpat#262 Fix extraction of namespace prefixes from XML names;
  XML names with multiple colons could end up in the
  wrong namespace, and take a high amount of RAM and CPU
  resources while processing, opening the door to use for denial-of-service attacks
• Add build condition libexpat/libexpat#227 Autotools: Add --without-examples and --without-tests
  Full changelog is available here.

from bitcoin/bitcoin#16270