m68k: pgtable: Add missing #include <asm/page.h>

When just including <linux/pgtable.h>:

include/asm-generic/pgtable-nop4d.h:9:18: error: unknown type name ‘pgd_t’
9 | typedef struct { pgd_t pgd; } p4d_t;
| ^~~~~

Make <asm/pgtable.h> self-contained by including <asm/page.h>.

Reported-by: Jani Nikula <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Jani Nikula <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/af9[e-mail]

arch/m68k/include/asm/pgtable.h

···
 #ifndef __M68K_PGTABLE_H
 #define __M68K_PGTABLE_H
 
+#include <asm/page.h>
+
 #ifdef __uClinux__
 #include <asm/pgtable_no.h>
 #else