drisw: Fix build on Android Nougat, which lacks shm (v2)

In commit cf54bd5e, dri_sw_winsys.c began using <sys/shm.h> to support
the new functions putImageShm, getImageShm in DRI_SWRastLoader. But
Android began supporting System V shared memory only in Oreo. Nougat has
no shm headers.

Fix the build by ifdef'ing out the shm code on Nougat.

Fixes: cf54bd5e "drisw: use shared memory when possible"
Reviewed-by: Dave Airlie <airlied@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@gmail.com>
(cherry picked from commit aaa41cd2)