lib: fix build failure in CONFIG_DEBUG_VIRTUAL test

lib: fix build failure in CONFIG_DEBUG_VIRTUAL test

On several arches, virt_to_phys() is in io.h

Build fails without it:

  CC      lib/test_debug_virtual.o
lib/test_debug_virtual.c: In function 'test_debug_virtual_init':
lib/test_debug_virtual.c:26:7: error: implicit declaration of function 'virt_to_phys' [-Werror=implicit-function-declaration]
  pa = virt_to_phys(va);
       ^

Fixes: b2dfaa4acf0b ("lib: add test module for CONFIG_DEBUG_VIRTUAL")
CC: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>