include: Make headers more self-contained

Back in 2016, we discussed[1] rules for headers, and these were
generally liked:

1. Have a carefully curated header that's included everywhere first.  We
   got that already thanks to Peter: osdep.h.

2. Headers should normally include everything they need beyond osdep.h.
   If exceptions are needed for some reason, they must be documented in
   the header.  If all that's needed from a header is typedefs, put
   those into qemu/typedefs.h instead of including the header.

3. Cyclic inclusion is forbidden.

This patch gets include/ closer to obeying 2.

It's actually extracted from my "[RFC] Baby steps towards saner
headers" series[2], which demonstrates a possible path towards
checking 2 automatically.  It passes the RFC test there.

[1] Message-ID: <87h9g8j57d.fsf@blackfin.pond.sub.org>
    https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg03345.html
[2] Message-Id: <20190711122827.18970-1-armbru@redhat.com>
    https://lists.nongnu.org/archive/html/qemu-devel/2019-07/msg02715.html

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20190812052359.30071-2-armbru@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>

include/hw/char/escc.h

@@ -3,6 +3,7 @@
 
 #include "chardev/char-fe.h"
 #include "chardev/char-serial.h"
+#include "hw/sysbus.h"
 #include "ui/input.h"
 
 /* escc.c */

include/hw/char/xilinx_uartlite.h

@@ -15,6 +15,8 @@
 #ifndef XILINX_UARTLITE_H
 #define XILINX_UARTLITE_H
 
+#include "hw/sysbus.h"
+
 static inline DeviceState *xilinx_uartlite_create(hwaddr addr,
                                         qemu_irq irq,
                                         Chardev *chr)