h8300/pgtable: add missing #include <asm-generic/pgtable.h>

Fix the h8300 build error:

  kernel/sched/core.c: In function 'context_switch':
  kernel/sched/core.c:2061:2: error: implicit declaration of function 'arch_start_context_switch' [-Werror=implicit-function-declaration]
Signed-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/h8300/include/asm/pgtable.h

@@ -70,4 +70,7 @@ extern int is_in_rom(unsigned long);
 #define	VMALLOC_END	0xffffffff
 
 #define arch_enter_lazy_cpu_mode()    do {} while (0)
+
+#include <asm-generic/pgtable.h>
+
 #endif /* _H8300_PGTABLE_H */