Bug 1760244 - Make badProtocolInclude syntactically valid. r=nika

The Python IPDL frontend checks for includes before checking if the
entire file is syntactically valid. However, my Rust IPDL front end
does it later, so it ends up failing in a syntax error before
noticing the invalid include. I think the basic issue is that this
file wasn't updated when async was made mandatory, because it didn't
affect the results. Let's just fix the file so it only tests one
thing.

Differential Revision: https://phabricator.services.mozilla.com/D141445

ipc/ipdl/test/ipdl/error/badProtocolInclude.ipdl

@@ -5,5 +5,5 @@ include protocol IDONTEXIST;
 // error: nonexistent protocol ^^^
 
 protocol badProtocolInclude {
-child: Msg();
+child: async Msg();
 };