Include less of the generated modular QAPI headers

In my "build everything" tree, a change to the types in
qapi-schema.json triggers a recompile of about 4800 out of 5100
objects.

The previous commit split up qmp-commands.h, qmp-event.h, qmp-visit.h,
qapi-types.h. Each of these headers still includes all its shards.
Reduce compile time by including just the shards we actually need.

To illustrate the benefits: adding a type to qapi/migration.json now
recompiles some 2300 instead of 4800 objects. The next commit will
improve it further.

Backports commit 9af2398977a78d37bf184d6ff6bd04c72bfbf006 from qemu

qemu/include/qapi/qmp/qobject.h

@@ -32,9 +32,7 @@
 #ifndef QOBJECT_H
 #define QOBJECT_H
 
-#include <stddef.h>
-#include <assert.h>
-#include "qapi-types.h"
+#include "qapi-builtin-types.h"
 
 struct QObject {
     QType type;

qemu/include/qapi/visitor.h

@@ -14,10 +14,7 @@
 #ifndef QAPI_VISITOR_H
 #define QAPI_VISITOR_H
 
-#include "qemu/typedefs.h"
-#include "qapi-types.h"
-#include "qapi/error.h"
-#include <stdlib.h>
+#include "qapi-builtin-types.h"
 
 /*
  * The QAPI schema defines both a set of C data types, and a QMP wire