svga: fix build on FreeBSD
MADV_HUGEPAGE only exists on Linux:
../src/gallium/winsys/svga/drm/vmw_screen_ioctl.c:697:41: error: use of undeclared identifier 'MADV_HUGEPAGE'
(void) madvise(map, region->size, MADV_HUGEPAGE);
(btw, this driver can be used on FreeBSD, we do have the vmwgfx kernel driver ported. we have some concurrency bug but it has worked on single-core VMs.)