util: Add os_get_page_size support for macOS.

Fix build error on macOS.

src/util/os_misc.c:352:2: error: unexpected platform in os_sysinfo.c
#error unexpected platform in os_sysinfo.c
 ^

Fixes: ("cdf3a6a8 util: Add os_get_page_size query") Signed-off-by: Vinson Lee vlee@freedesktop.org

cc: @jenatali