ALSA: core: Fix build error due to missing PAGE_SIZE

The recent refactoring of memalloc stuff removed the inclusion of
asm/page.h for simplicity, but it turned out this caused a compile
error due the lack of PAGE_SIZE definition on some architectures.
Do a partial revert for recovering from that.

Fixes: 37af81c5 ("ALSA: core: Abstract memory alloc helpers")
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/202106101858.PfXMMuAa-lkp@intel.com
Link: https://lore.kernel.org/r/20210610110935.10393-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/memalloc.h

@@ -9,8 +9,9 @@
 #ifndef __SOUND_MEMALLOC_H
 #define __SOUND_MEMALLOC_H
 
+#include <asm/page.h>
+
 struct device;
-struct page;
 struct vm_area_struct;
 
 /*