mm/process_vm_access: Add missing #include <linux/compat.h>

With e.g. m68k/defconfig:

    mm/process_vm_access.c: In function ‘process_vm_rw’:
    mm/process_vm_access.c:277:5: error: implicit declaration of function ‘in_compat_syscall’ [-Werror=implicit-function-declaration]
      277 |     in_compat_syscall());
	  |     ^~~~~~~~~~~~~~~~~

Fix this by adding #include <linux/compat.h>.

Reported-by:  <noreply@ellerman.id.au>
Reported-by: damian <damian.tometzki@familie-tometzki.de>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Fixes: 38dc5079 ("Fix compat regression in process_vm_rw()")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>