m68k: mm: Move paging_init() to common <asm/pgtable.h>

When building with W=1:

    arch/m68k/mm/motorola.c:414:13: warning: no previous prototype for ‘paging_init’ [-Wmissing-prototypes]
      414 | void __init paging_init(void)
	  |             ^~~~~~~~~~~
    arch/m68k/mm/sun3mmu.c:36:13: warning: no previous prototype for ‘paging_init’ [-Wmissing-prototypes]
       36 | void __init paging_init(void)
	  |             ^~~~~~~~~~~

Fix this by consolidating the multiple prototypes into the common
<asm/pgtable.h>.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/1b03fde54f205e972e19959b8e335022205d538c.1694613528.git.geert@linux-m68k.org

arch/m68k/include/asm/pgtable.h

 /* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __M68K_PGTABLE_H
+#define __M68K_PGTABLE_H
+
 #ifdef __uClinux__
 #include <asm/pgtable_no.h>
 #else
 #include <asm/pgtable_mm.h>
 #endif
+
+#ifndef __ASSEMBLY__
+extern void paging_init(void);
+#endif
+
+#endif /* __M68K_PGTABLE_H */

arch/m68k/include/asm/pgtable_no.h

@@ -28,7 +28,6 @@
 #define PAGE_READONLY	__pgprot(0)
 #define PAGE_KERNEL	__pgprot(0)
 
-extern void paging_init(void);
 #define swapper_pg_dir ((pgd_t *) 0)
 
 /*

arch/m68k/kernel/setup_mm.c

@@ -107,8 +107,6 @@ EXPORT_SYMBOL(isa_sex);
 
 #define MASK_256K 0xfffc0000
 
-extern void paging_init(void);
-
 static void __init m68k_parse_bootinfo(const struct bi_record *record)
 {
 	const struct bi_record *first_record = record;