Makefile: Remove generated headers on clean

Running `make distclean' followed by a new out-of-tree build would fail
due to stale generated QMP headers in the tree.

Commit 611b7273 (Makefile: remove more
generated files on clean) made sure generated sources are removed.

Also remove generated headers introduced in commit
e3193601 (qapi: use middle mode in QMP
server).
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

Makefile

@@ -213,6 +213,7 @@ clean:
 	rm -f trace.c trace.h trace.c-timestamp trace.h-timestamp
 	rm -f trace-dtrace.dtrace trace-dtrace.dtrace-timestamp
 	rm -f trace-dtrace.h trace-dtrace.h-timestamp
+	rm -f $(GENERATED_HEADERS)
 	rm -f $(GENERATED_SOURCES)
 	rm -rf $(qapi-dir)
 	$(MAKE) -C tests/tcg clean