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 <jani.nikula@intel.com>
Closes: https://lore.kernel.org/r/878r2uxwha.fsf@intel.comSigned-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/af9e22b878f59223adb593f5bbd5b61432120010.1709898638.git.jani.nikula@intel.com

arch/m68k/include/asm/pgtable.h

@@ -2,6 +2,8 @@
 #ifndef __M68K_PGTABLE_H
 #define __M68K_PGTABLE_H
 
+#include <asm/page.h>
+
 #ifdef __uClinux__
 #include <asm/pgtable_no.h>
 #else