tests/Makefile.include: Fix missing test-qapi-emit-events.[ch]

tests/Makefile.include: Fix missing test-qapi-emit-events.[ch]

Commit 5d75648b56 "qapi: Generate QAPIEvent stuff into separate files"
added tests/test-qapi-emit-events.[ch] to the set of generated files,
but neglected to update tests/.gitignore and tests/Makefile.include.
Commit a0af8cee3c "tests/.gitignore: ignore test-qapi-emit-events.[ch]
for in-tree builds" fixed the former.  Now fix the latter.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191120182551.23795-3-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>