cmake clang/compiler_flags.cmake: Re-enable -Wunknown-warning-option

All warnings in the code base have been resolved.

Signed-off-by: Tom Hughes <tomhughes@chromium.org>

Author: thughes

cmake/compiler/clang/compiler_flags.cmake

@@ -108,7 +108,6 @@ check_set_compiler_property(PROPERTY warning_extended
                             -Wno-unused-function
                             -Wno-initializer-overrides
                             -Wno-section
-                            -Wno-unknown-warning-option
                             -Wno-unused-variable
                             -Wno-format-invalid-specifier
                             -Wno-gnu