Gallium: Fix shared memory segment leak

Commit abe6d750e5 caused shared memory
segments to be leaked. We need to mark shared memory segments for
deletion upon construction.

Fixes: abe6d750e5 xlib: fix glXDestroyContext in Gallium frontends
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/9425
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24556>

src/gallium/winsys/sw/xlib/xlib_sw_winsys.c

@@ -138,6 +138,7 @@ alloc_shm(struct xlib_displaytarget *buf, unsigned size)
       return NULL;
    }
 
+   shmctl(shminfo->shmid, IPC_RMID, 0);
    shminfo->readOnly = False;
    return shminfo->shmaddr;
 }