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/timer/allwinner-a10-pit.h

@@ -2,6 +2,7 @@
 #define ALLWINNER_A10_PIT_H
 
 #include "hw/ptimer.h"
+#include "hw/sysbus.h"
 
 #define TYPE_AW_A10_PIT "allwinner-A10-timer"
 #define AW_A10_PIT(obj) OBJECT_CHECK(AwA10PITState, (obj), TYPE_AW_A10_PIT)

include/hw/timer/i8254_internal.h

@@ -27,6 +27,7 @@
 
 #include "hw/hw.h"
 #include "hw/isa/isa.h"
+#include "hw/timer/i8254.h"
 #include "qemu/timer.h"
 
 typedef struct PITChannelState {

include/hw/timer/m48t59.h

@@ -1,6 +1,8 @@
 #ifndef HW_M48T59_H
 #define HW_M48T59_H
 
+#include "exec/hwaddr.h"
+#include "hw/irq.h"
 #include "qom/object.h"
 
 #define TYPE_NVRAM "nvram"

include/hw/timer/mc146818rtc_regs.h

@@ -25,6 +25,8 @@
 #ifndef MC146818RTC_REGS_H
 #define MC146818RTC_REGS_H
 
+#include "qemu/timer.h"
+
 #define RTC_ISA_IRQ 8
 
 #define RTC_SECONDS             0

include/hw/timer/xlnx-zynqmp-rtc.h

@@ -28,6 +28,7 @@
 #define HW_TIMER_XLNX_ZYNQMP_RTC_H
 
 #include "hw/register.h"
+#include "hw/sysbus.h"
 
 #define TYPE_XLNX_ZYNQMP_RTC "xlnx-zynmp.rtc"