Add -Werror=unknown-warning-option to CLANG_FLAGS

[nathanchance]

Supplements #511.

Patch sent: https://lore.kernel.org/lkml/20190611184331.44242-1-natechancellor@gmail.com/

[nathanchance]

Patch is in -next: https://git.kernel.org/next/linux-next/c/589834b3a0097a4908f4112eac0ca2feb486fa32

[nathanchance]

Merged into mainline: https://git.kernel.org/torvalds/c/589834b3a0097a4908f4112eac0ca2feb486fa32