It's a bug that has since been fixed by kernel commit
2220fc1ab363e6fab1f321430d69be17a8b92bd7 ("uapi: add missing install
of dma-buf.h"). The header was originally added in 4.10, and the fix
commit is in 4.11-rc1.I guess we just need to hack around it in sys-kernel/linux-headers-4.10.
