Build: Add docs/*.8.xml to .gitignore

These are intermediate files that are generated during man page
creation. They may be left behind in the event of an xmllint error, for
example.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>

Author: nrwahl2

.gitignore

@@ -24,6 +24,7 @@ booth-*.tar*
 conf/booth*.service
 docs/*.8
 docs/*.8.html
+docs/*.8.xml
 script/service-runnable
 src/b_config.h.in
 src/b_config.h