m68k: apollo: Add and use "apollo.h"

When building with W=1:

    arch/m68k/apollo/dn_ints.c:43:13: warning: no previous prototype for ‘dn_init_IRQ’ [-Wmissing-prototypes]
       43 | void __init dn_init_IRQ(void)
	  |             ^~~~~~~~~~~

Fix this by introducing a new header file "apollo.h" for holding the
prototypes of functions implemented in arch/m68k/apollo/.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/d78eceb83b8dd1931be1789204898060664e23f6.1694613528.git.geert@linux-m68k.org

arch/m68k/apollo/apollo.h

@@ -0,0 +1,4 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+/* dn_ints.c */
+void dn_init_IRQ(void);

arch/m68k/apollo/config.c

@@ -17,6 +17,8 @@
 #include <asm/machdep.h>
 #include <asm/config.h>
 
+#include "apollo.h"
+
 u_long sio01_physaddr;
 u_long sio23_physaddr;
 u_long rtc_physaddr;
@@ -27,7 +29,6 @@ u_long timer_physaddr;
 u_long apollo_model;
 
 extern void dn_sched_init(void);
-extern void dn_init_IRQ(void);
 extern int dn_dummy_hwclk(int, struct rtc_time *);
 static void dn_dummy_reset(void);
 #ifdef CONFIG_HEARTBEAT

arch/m68k/apollo/dn_ints.c

@@ -5,6 +5,8 @@
 #include <asm/traps.h>
 #include <asm/apollohw.h>
 
+#include "apollo.h"
+
 static unsigned int apollo_irq_startup(struct irq_data *data)
 {
 	unsigned int irq = data->irq;